A few thoughts. The St Petersburg Metro is very deep, I think people would have survived.
The direct radiation from the bomb, especially Ɣ-rays is immense, but the land is not uninhabitable afterwards. The short-lived radionuclides are much more radioactive than long-lived ones, but they disappear quite quickly. In the 1st few weeks, 95% of the problem is 131I, (because an Iodine-deficient individual's body is actively trying to absorb Iodine from the environment.) & absorption can be blocked by saturating your body with Iodine before you arrive. With a half-life of 8 days, 131I is <1000 after 3 months, < 1 trillion within 12 months.
I would expect Finland, Estonia & Latvia, Belarus & Ukraine to be generous in helping themselves to any territory about which they might have a vague historic claim; they will ask the handful of remaining people, but most will prefer to remain part of a functioning state.
Reparations will presumably mean the UN managing royalties from resources & paying reparations from these funds.
There have been a number of proposals to increase the number of permanent members of the security council, but new members wouldn't have a veto. India, Brazil, Japan & Germany have been suggested. IOTL, China would probably veto Japan & India, adding Germany without those 2 makes it too eurocentric & then the proposal collapses. Nonetheless, it was possible to change the representation of China. I expect that ITTL, the various Russian factions compete for the UN seat, offering to give up the veto.
