wait... you cant ever form union over poland if they took union with lithuania and get elective monarchy... are you not using dlc or someting?
You can, they change government later on though. Either to a republic or to a non-elective monarchy. It used to be to an Absolute Monarchy but they removed it, not sure what it goes to now- but they have a civil war. Monarchy vs the Sejm essentially.
He didn't get it on monarch death, that's for sure though. It's also unlikely Poland didn't form the Commonwealth unless Lithuania got too big, by the time the civil war happens, you're usually the Commonwealth.
Also OP: You won't be able to PU Russia. You have no RM with them either, unless you had one before they got PU'd as they can stay around for some time, maybe that only applies to the person who PU'd Russia-but this Marriage will not gain you anything.
If you can't PU Circassia through a Claim Throne war- then you won't be able to get both them and Russia in a PU. It will have to be done separately, and you'll have to wait for Russia to be independent then try get on their throne.