> So far, the policy differences between the different far-right parties in Europe hasn't really prevented them from cooperating
They literally have. France's RN officially stopped collaborating with the German AfD over the latter being too "extreme". Now they sit in different fractions of the EU parliament.
That is the one such development I can think of in recent history. We'll see if this becomes more common as far-right parties become more mainstream, but I'm not holding my breath.
On the other hand, I've seen US-influenced and Russia-influenced movements happily cooperate for years now. There's been some tensions over which side to support when the war in Ukraine broke out, but so far it hasn't prevented these same groups clashing on this specific issue to cooperate on other issues.
They literally have. France's RN officially stopped collaborating with the German AfD over the latter being too "extreme". Now they sit in different fractions of the EU parliament.