For your third question see my post here and the subsequent replies:
I don't really know what collections are exactly on OpenSea. I suggest browsing OpenSea documentation and potentially asking about that in their Discord server.
For batch minting, we don't have a simple solution. But you can write a contract and call _mint in a loop in your constructor. This will be more expensive than it technically needs to be, but as I there's no simple cheaper alternative right now.