Happy to help!

Yes, CoinOS and Primal wallets would be independent from one another. You will need to choose which one you want to receive your zaps to. I would recommend whichever one you plan to do most of your zapping from. You CAN change it later, if you see you are using a different wallet more than you had expected you would.

In order to change which wallet you are receiving zaps to, you will need to go to your profile settings and change the lightning address from DannyConnolly@primal.net to whatever you set up with CoinOS as your address.

You absolutely can transfer back and forth between both wallets whenever you need to. Lightning fees are generally small and reasonable.

Whenever you are using Primal on your iOS device, you will always be zapping from your Primal wallet balance.

Once you have CoinOS connected to your Alby extension, you will always be zapping from your CoinOS balance on any Nostr web app.

Whichever lightning address you have listed in your profile will always be where you receive zaps from other nostriches.