Several of your questions are covered by:
Google for the answer on castellated holes if you like, but the answer is, you don’t need them.
Every individual object (panel, PCB) is a separate submission.
Default thickness is fine.
I don’t know what stencil “Electropolishing” and “Framework” are but am confident they are not needed.
If I have leftover PCBs/panels of my own design I sell them on Tindie (see BST topic). If it’s someone else’s design it depends on whether they made it open source or more generally how they feel about it. Trading for others’ extras is also an option.
(I even order more sometimes if I think others will want them. For small boards, under 10 cm each way, ten boards cost only a dollar more than five.)
JLCPCB will charge you extra if they think a “board” you submit is really two or more boards to be cut apart. It might still be cheaper.