There should definitely be a Nabbit% since any% speedrunners might have to default to using Nabbit as he might be faster in most stages for damage boosting.

Yes, there will be a checkmark on your save file when you have everything as well as the medals which track your completion for each collectable. Here's all the medals:

Medal 1 - Beat the Game

Medal 2 - All Wonder Seeds

Medal 3 - All 10 Coins

Medal 4 - All tops of flagpoles

Medal 5 - All standees

Medal 6 - All other collectables and complete the final-final challenge