Actually, what you are describing here is what Coinos was (and Coinos Classic still is), or at least comes very close. Basically a three-way bridge between BTC, L-BTC, and Lightning where you can send one in, and take any of the other two out.

Not sure you even need HC for that. As a wallet operator you just maintain three nodes, each with their own balance.

You could scan a LN invoice, and "pay" with your Liquid balance - the booking is done behind the scenes by the wallet (debit Liquid, credit Lightning, dedit Lightning for final payment of the invoice). You can apply a small fee (the bridging fee in your example) on top, accounting for the fact that more people want Lightning than Liquid.

I think pretty much all of that is in the Coinos codebase already