The UK doesn't cease to exist though, it just shrinks. Plus the USSR fragmenting and Russia (as the main constituent part, the nuclear power and the country the independent republics were happy to acknowledge as the continuation of the USSR) becoming the successor state is pretty well-established precedent for what happens when states fragment, whose legitimacy Russia probably doesn't want to contest too strongly...

General disruption in the UK would help the Iranian government a little, but I managed to click on one of the accounts before it was suspended, and its most popular tweets received very interaction (and were pretty banal statements of independence support indistinguishable from stuff thousands of completely normal Scottish people posted) I assume their attempts to seed wilder rumours were low effort and had very little success.

Russia didn't lose its veto when the USSR collapsed and neither would the UK lose it in such a case. If the UK was in danger of losing its veto it would never allow Scottish independence.

It doesn’t matter whether independence is realized, what Iran wants is more time and effort spent on domestic disagreement, so less is available to support international engagement.

This is a tool of international competition and the U.S. and U.K. have been trying to do it to Iran (and others) for even longer than the reverse.

If Russia kept the soviet UNSC seat when the Soviets collapsed then surely the UK keeps its seat if Scotland leaves.

All that would happen would be Scotland would lose its influence over the veto.

The UK is a lot more then Scotland + England. The Welsh, northern Irish and Isle of Manx would like to have a word with you, to name a few.