> the Soviet-German military co-operation started after the Rapallo treaty of 1922 and waned after 1932

And the precise reason the co-operation "waned" was the rise of the Nazis. IOW, what happened was the complete opposite of what you claimed happened.

> it is quite well documented that the German re-armament, forbidden by the Treaty of Versailles, was largely made possible by this co-operation with USSR.

Pointing out co-operation between the USSR and Germany (not the Nazis) is one thing. Singling it out, in order to claim German re-armament was "largely made possible" due to the USSR simply doesn't hold up, and conveniently ignores the role of many other countries in German re-armament.

For example:

In the important field of submarine warfare, Germany
found a safe haven in Finland. [...] In addition to
Finland, countries such as Japan, Spain, The Netherlands,
Turkey, and Sweden helped German shipyards and the navy
[...]
In Germany's web of covert contacts, *Finland played
a central role*.

From "Finnish-German Submarine Cooperation 1923-35" by Jason Lavery.

> to be reconciled at the Molotov-Ribbentrop Pact.

The motivation behind which was a realization by Stalin that the USSR was on its own, and a requirement to delay any war with Hitler in order to strengthen the Soviet military.

Attempts by Stalin to form an anti-Nazi pact with France and Britain had failed, at least in part because Poland flatly refused to allow the Red Army (the only army realistically capable of aiding Poland) to defend Polish soil if the Nazis attacked.

We can blame Stalin for a lot of things, but I'm not sure what alternative he had to a non-aggression pact with Hitler, other than simply watching Panzer divisions roll up to the Soviet border.

> The Germans visiting Soviet extermination camps is documented at least in "Kremlin kellot" by Arvo Poika Tuominen (a Finnish Communist who worked for Komintern in 1933-1939 and spent quite some time observing the camps at the Baltic-White Sea Canal works and Solovki prison.)

From Tuominen's English Wikipedia page:

Research by Finnish historian Kimmo Rentola has exposed
[the falsehood of Tuominen's claim to be against the
Soviets during the Soviet-Finland war]. When the war
started, Tuominen was initially enthusiastic of the war
in expectation of a quick Soviet victory.

So the source of the claim of Nazi-Soviet collaboration on death camps is someone who supported the USSR against Finland even after supposedly witnessing the "extermination camps" in the USSR?

A man who, when he realized he would not become part of a Soviet quisling regime in Finland, switched to being overtly and conspicuously anti-Communist, with a clear motivation to lie or exaggerate the crimes of his former comrades.

> It's a pretty chilling read; the canal works were where the basic research for extermination via forced labour was done.

Soviet gulags were horrible enough places without falsely claiming them to be something they are not. And they were no more extermination camps than Indian "reservations" were, or British camps in South Africa, or the East Karelian concentration camps run by Finland.

> Solzhenitsyn then later provided a more well-known description of the GULAG.

Where does Solzhenitsyn claim gulags were "extermination camps"? In fact, where does any credible source?

Further, where does any credible source provide evidence that the Nazis received help or ideas from the USSR for their extermination programme?

The precursor to the Final Solution was in fact the Nazi eugenic and euthanasia programmes, and it's no secret they were inspired by and based on programmes in the USA (not USSR) in the early 20th century.