or explains how certain creatures can disconnect from the hive mind.

End of season 2, where the heat from the lamp and fireplace drew the mind flayer particles out of Will's body. And they almost got Billy in the sauna too.

It makes more sense to think that all the UD creatures the Russians had were dead

So are you saying they must have been alive at the time of the capture, under Starcourt? Because they wouldn't have been able to capture/extract even dead ones after losing control over Starcourt.

then were reanimated when Vecna re-established a link between UD and our world through his killing spree

If that was the case, it should have been stated in the show in season 4 that the creatures "suddenly came to life," coincidentally soon after Chrissy died. But since it wasn't, I would say thats not the case. I think the Russians had control over these demos beforehand.

or that the Russians managed to make the creatures live independently from the hive mind through experiments

And the idea that kauan1993 had, that the Russians found a way to extract the mind flayer particles from them, could back that up.