I'm not sure to understand "your understanding".. you will need to elaborate more..
About your questions:
1) No you can have just one Persistence Unit and use the Local Task Service
2) If you use just one Persistence Unit you don't need to care about that.. JTA or Resource Local are for cordinating more than one Transactional Resourece inside the same transaction
3) Yes
4) no, it's up to you, you can use just a single schema..
Cheers