You can do either. Setup with a new key, or add an existing.

Once setup you should only ever need your seed phrase as a last resort or during updates (check you have access to it before upgrading as a precaution).

Depends on the hardware wallet, however you can export for xpublic key and it can tell you balances without any secret keys - just know that the xpublic key should be kept private for your privacy, but you can’t steal funds using it.

Plus the wallets come with instructions. I’d just follow those.