If LN is off-chain, and the transactions are all contracts that aren't published by miners... is it still redundant? Bitcoin is infallible only because you have redundancy between millions of miners who can confirm transactions because they all match up with everyone's ledgers.

But if LN is not mined, then it stands to reason that there's some kind of central system or some kind of system where the transactions are only in existence between two parties.

Doesn't that simply create an exploitable hole that allows for hacks and cheats that would just weaken bitcoin overall?