1885 article by Friedrich Engels accusing Russia of involvement in London bombings

but I think Marx thought that tsarist Russia played a considerable role in propping up reactionary forces in Europe.

iirc he believed in Urquhart's theory that Lord Palmerston was a secret Russian agent (an anti-Marxist blogger did a post on it).
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Thanks for that 2007 piece on the context of Engels's article. Apparently there is a Russian translation of it (the book which the Russian spokeswoman shows).
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Now that we're on the subject, any thoughts about the Skripal affair? I heard one speculation that it was an inside job to let Boris Johnson jump the gun and discredit himself (after his claim that Russia did it will turn out to be false), and so possibly weaken the Brexit camp inside the Tory party.

Ah there does already exist an English translation, as someone mentioned on twitter a few days ago. MECW, Vol. 26 (1990), pp. 292–5

Real Imperial Russian Privy Dynamiters

Everybody knows that the Russian government is using every
means at its disposal to arrive at treaties with the West European
states for the extradition of Russian revolutionaries who have fled
the country.
Everybody also knows that its overriding concern is to obtain
such a treaty from England.
And the final thing that everybody knows is that Russian
officialdom will shrink at nothing if only it leads to the desired
end.
Very well then. On January 13, 1885 Bismarck concludes an
agreement with Russia, which provides for the extradition of every
Russian political refugee the moment Russia sees fit to accuse him
of being a prospective regicide, or prospective dynamiter.1
On January 15 Mrs Olga Novikov issued an appeal to England
in the Pall Mall Gazette, the selfsame Mrs Novikov who in 1877
and 1878, before and during the war against the Turks, so
magnificently duped the noble Mr Gladstone in the interests of
Russia.
In it England is exhorted no longer to tolerate people
such as Hartmann, Kropotkin and Stepniak conspiring on English
soil "to murder us in Russia", especially now that dynamite has
become such a burning issue for the English themselves. And, she
remarks, is Russia asking any more of England with respect to
Russian revolutionaries than England itself is now obliged to ask
of America with respect to Irish dynamiters?
On the morning of January 24 the Prusso-Russian treaty is
published in London.
And on January 24 at 2 o'clock in the afternoon, three dynamite
explosions go off in London within the space of a quarter hour,
and they cause more damage than all the earlier ones taken
together, wounding at least seven people, and according to other
sources eighteen.
The timing of these explosions is too opportune not to raise the
question—Whose interests do they serve? Who has most to gain
from these otherwise pointless shots of terror aimed at nobody in
particular, to which not only lower-ranking policemen and
bourgeois fall victim but also workers and their wives and
children? Who? The few Irishmen who were driven to desperation
partially because of the brutality of the English government
during their imprisonment, and who are assumed to have planted
the dynamite? Or, on the other hand, the Russian government
which cannot achieve its end—the extradition treaty—without
putting the government and people of England under the most
extreme pressure, pressure so great that it whips up public
opinion in England into a blind rabid rage against the dynamiters?
When the Polish refugees with very few exceptions, would not
lower themselves, at the behest of the Russian diplomatic service
and the police, to forge Russian banknotes, the Russian govern-
ment sent agents abroad, including privy councillor Kamensky, to
goad them into doing it, and when this too failed Messrs
Kamensky and associates were obliged to forge Russian banknotes
themselves. For a further detailed account see the pamphlet
The Counterfeiters or the Agents of the Russian Government,
Geneva, H. Georg, 1875.2 —The police forces
of Switzerland and London, and probably of Paris as well, can tell a tale or two about how, in tracking down the Russian forgers, their inquiries finally led them
to people whom the Russian embassies would steadfastly refuse to have prosecuted.
The history of the Balkan peninsula during the past one
hundred years sheds enough light on the abilities of Russian
officialdom in removing troublesome individuals by means of
poison, the dagger, etc. I need refer only to the well-known
Histoire des principautés danubiennes by Elias Regnault, Paris, 1855.
The Russian diplomatic service constantly has at its disposal agents
of all kinds, including the kind that are used to commit infamous
deeds and then disowned.
I do not hesitate, for the time being to lay the blame for the
explosions in London on January 24, 1885 at the door of the
Russians. Irish hands may have laid the dynamite, but it is more
than probable that a Russian brain and Russian money were
behind it.
The means of struggle employed by the Russian revolutionaries
are dictated to them by necessity, by the actions of their opponents
themselves. They must answer to their people and to history for
the means they employ. But the gentlemen who are needlessly
parodying this struggle in Western Europe in schoolboy fashion,
who are attempting to bring the revolution down to the level of
Schinderhannes, who do not even direct their weapons against real
enemies but against the public in general, these gentlemen are in
no way successors or allies of the Russian revolutionaries, but
rather their worst enemies. Since it has become clear that nobody
apart from Russian officialdom has any interest in the success of
these heroic deeds, the only question that remains to be asked is
which of them were coerced and which of them volunteered to
become the paid agents of Russian tsarism.

London, January 25, 1885
Frederick
Engels

First published in
Der Sozialdemokrat,

5, January 29, 1885

• 1See "Extradition by Russia and Prussia", The Times, No. 31352, January 24, 1885.—ed
• 2Published in Russian.

And are there any histories of the court cases surrounding the bombings that may have lead to more information being discovered or brought out about the bombings supposedly "carried out by Irish hands but financed by Russians" or whatever?

No, not that I can think of, and obviously the Russian spokeswoman brought up Engels' theory of Russian involvement as an example of unfounded Russia-blaming.

Of course liberals in the U.S. are exploiting any opportunity to paint their failure in the elections as *chiefly* the result of a Russian Plot

The existence of the "troll factory" in Russia was reported by a young journalist guy months prior the election, but I don't remember if they already tried to connect it to promotion of Trump then (tho of course Trump was already portrayed as a Russian agent throughout the election campaign). It seems Alex Jones made the point that it is possible that Trump hired that company to work for him (i.e. it was not Putin ordering the company), but that such a practice, i.e. politicians using the services of such type of companies abroad, is nothing unusual. It's comparable to how one can buy "likes/views" to boost up one's brand/profile thanks to some click-factory (with thousands of phones, whole day clicking) in China or anywhere abroad.

This week's news briefing features a lengthy historical overview of the crimes of British imperialism around the globe:

starts at min. 41 (last about an hour):
https://www.youtube.com/watch?v=j8cjrgPc4Ew

For example in spring 1920 the British used chemical weapons (mustard gas) against Iraq rebellion with 6,000 to 10,000 killed (mentioned at the 1h mark in the video).

So Churchill ordered the use of mustard gas in Iraq (and the RAF wanted to use it), but it appears the bombs weren't ready yet. Now, if I were a disagreeable person (and adopt the logic of some libcom users around here), I should start calling you an apologist for British imperialist crimes merely for pointing out that there is no evidence of actual mustard gas bombing (but also certainly for linking to a text on a Churchill website, which mentions that mustard gas had only a 2% fatality rate, basically harmless stuff in other words). Or how about I chose to adopt your logic, that absence of evidence does not constitute evidence of absence.
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But let's turn to the Skripal case at hand. I'm still not sure whether there is an actual positive test that the frontdoor handle was poisoned with nerve agent (first told by Hamish de Bretton Gordon, incidentally also a pro-war Syria guy). That's what the press makes you believe.

All I'm certain about is that samples were tested from the Skripals' blood. And these seem taken some two weeks afterwards. And that the nerve agent was found in high purity raises questions, since normally the stuff is broken down a bit in the body.

Gilbert's book (which published the Churchill quotes) dates from 1977, Douglas's article (arguing against the claim) dates from 2009, but perhaps curiously the Russian Foreign Ministry's briefing mentions declassified archives from 2014:

One more interesting fact. According to the British national archives declassified in 2014, the British authorities made wide use of chemical weapons to put down the Arab rebellion in Mesopotamia (present-day Iraq) in the spring of 1920. Winston Churchill as Britain’s Secretary of State for War supported “the use of gas against uncivilised tribes.” According to archives, Churchill ordered the use of thousands of mustard gas shells against the rebels. The anti-British rebellion in Iraq claimed between 6,000 and 10,000 lives, according to various sources, a negligible number from London’s point of view compared to other regions.

http://www.mid.ru/en/foreign_policy/news/-/asset_publisher/cKNonkJE02Bw/content/id/3178301