Z will get a key to unspent 100, a (new) key to unspent 295 is send back to the sender (A) and the miner will get a key to 5 (as a fee).
No.
This is incorrect.
Keys are NOT sent.
Each user creates their own keys. Z will generate a bitcoin address from his PUBLIC key and will provide the bitcoin ADDRESS to the sender (A). A will NOT send ANY keys to Z at all.
I don't know for sure
Then please don't give bad advice in the Technical sub-forums. You will cause other people to lose money if they believe the false things that you say. Do you want to be responsible for someone else losing thousands of dollars?