(2) Cross-chain atomic swaps work with or without Lightning. https://arxiv.org/abs/2101.12332
(3) As long as you don't exchange your 'Lightning Bitcoin' funds for 'Lightning Litecoin' funds or something else, they are always 'redeemable' for on-chain Bitcoin funds by submitting a channel close transaction. They are always 'linked' to the channel funded with that type of coin on that type of chain. It's not that hard. It's just a different type of contract. You may be aware that the Bitcoin script language allows different types of transactions. LN channel open / close transactions are just transactions. It's no sacrilegious black magic from hell.
1. when a potential user is introduced by asking them to hand in fiat to a service and is given service database balance.. is that "bitcoin"?
Using keys to 'unlock' a script is actually what you do in every single Bitcoin transactions. Locking script and unlocking script. All the time. When you 'spend BTC' in a typical on-chain transaction, you actually lock Bitcoin behind a script that only the owner of the private key generating a given address, owns.
https://learnmeabitcoin.com/technical/p2sh
For the record:
I'm not answering for franky because it is pointless (explained it 100 times, he must be very old or something), but for anyone reading this with some intelligence and logic, who got confused by him. If you as a reader are interested in more details about the msats, search in this thread with the right keywords, I explained it all in detail: https://ninjastic.space/search?author=n0nce&content=msat&topic_id=5348270