Though it's still unknown on how they are going to fill these wallets. It seems to me they'll have wallets that contain an arbitrary ($1, $5, $10, etc) amount and they'll add them together and ship a subset of the keys to fill the exact amount from primary (buyer) to the secondary user (seller).

I suspect when you add funds in crypto to PayPal it'll be responsible for creating these "secondary wallets" for you, abstracted from the user (in the form of a simple balance). Same for the secondary user, when they receive a balance it'll most likely be shown as a simple balance.