An account is what you get on custodial services like centralized exchanges. Binance, for example. They hold all the keys and control the money. You own only the account. If you want to withdraw from that account, they will decide if that is ok or not. A wallet can be custodial or non-custodial. If it's non-custodial then you hold your own keys and can decide what to do with your bitcoin.

Your seed can derive one master private key. That master private key can then derive child private keys and eventually all your addresses.

If you generated a new wallet using Sparrow on a different computer, you would get a totally different master private key. Obviously, all other derived keys and addresses from that master key would also be different. You would also get a different master key from a wallet that you extended with a passphrase.

An xpub is an extended public key. From it you can generate all the addresses at that particular derivation path. Or from that particular account if you want. Software like Ledger Live calls each unique derivation path a new account. But the xpub isn't enough to allow you to use all the features of a wallet. It's missing the signing keys - the private keys.

You would be sending from addresses derived at a specific derivation path under the condition that you have the private keys that correspond to the addresses you are trying to send from. Like I said, the xpub alone won't give you that. The seed or extended private key will. Alternatively, you will need individual child private keys for each address.

Not by default because the xpub corresponds to addresses generated on a specific derivation path. The thing you call an account. The account contains the xpub for that derivation path but not for other accounts. What you could do is create a wallet with imported private keys from multiple other wallets (be it from Sparrow or other software). You could then fund and make transactions from the addresses derived from those private keys.