Yes. If you change your mind after you’ve played, you can pick those cards back up in the same turn as

long as you haven’t discarded yet.

No. Each set or run can only be extended.

Yes.

Yes.

The first card placed face up in the Discard pile from the deal should be treated as if the dealer played it. If it's an Action card, the dealer will benefit from the action. If it's a Wild card however, the next player can pick it up because the dealer has essentially discarded it.