we don’t know that. Their engines were pretty much a mystery to the federation until way into TNG and even then it was not addressed if dilithium was used or not.Point 1. Until DIS s3 decided to go with the Burn, it wasn't canon that Romulans used dilithium in their engines. That was my entire point about canon. It makes no sense to argue that it was part of canon AFTER the Burn was developed.
It must be pointed out that dilithium was extremely valuable in TOS but quite cheap to make in TNG, so it’s no surprise either way: while not using it would have been very notable in the 23rd century, by the 24th it would have been just a side note; on the other hand if they did use it it would have been equally unremarkable, like saying that an engine today relies on petroleum.
we don’t know that either.Point 2. The Romulans were only recluses against our side of their territory. And with enough internal space they could sustain plenty of interstellar trade.