Is anybody here familiar enough

No, the implementations are all larps borrowing open libraries they don't understand

in exchange for the payment of a lightning invoice

Ingest of that invoice requires other metadata

someone else could run a gateway connected to the same mint

The mint is the chokepoint, daisy chaining them just makes it worse

This is only possible for outgoing lightning

No shit, what else are the shitcoins good for?

Not sure what they are.

ECash relies on ignorance to proliferate

it's better than custodial lightning wallets

No it isn't, it is custodial completely, with worse overhead, and is an attack on Lightning's network effect as a payment spec