Good catch. We can assume I > B or better I -> infinity ( or n -> 0). Good think is we can test the final equation writing a rather simple simulation program.

Are you sure about:

It seems like you're assuming that all of the coins in a single UTXO will stake. When I stake with my wallet though, that's not the case - only a portion of my coins stake.

As wallet =/= UTXO (or address =/= UTXO) a portion of a wallet will stake, but whole UTXO.
It should be impossible to lock just a part of UTXO. You can check list of UTXOs with a command

listunspent

It's great to inspire and motivate each other!