I think what's confusing is credits vs votes. It's more expensive in credits for Alice to cast the first N votes than to cast the next N. However, for Bob those "next N" are his first N. So 2N votes costs fewer credits if Alice and Bob each cast N than if Alice casts 2N.

Also, Bob might end up with "leftover" credits he can't allocate to issues he cares about. So those credits might go unused in some sense.

Yes. In your first scenario P and Q get 10 votes each. In your second they get sqrt(75) + sqrt(25) = 13 point something.