I prefer him to be invincible. It's one of the things that makes the games more uniquely 'Wario'. Not being invincible also makes it harder to take advantage of the transformations (which is one of my favourite aspects of Wario Land) since on a...
Well, I'm pretty sure you don't need 10,000 coins on every level to do so. I suspect it'd just be beating all the bosses with all the treasure intact, though I've not unlocked it for a while.
As for how it differs... it's got way, way more...
I beat the game on hard mode thinking that was all I needed to do to unlock s-hard mode but I was wrong. I googled for a bit and there's a bunch of contradictory info on whether I need to 100% all collectables and get 10,000+ score on each...
Thanks for timelining this all out, this news got me wanting to do the same thing, also I'd like to add WL3 was added to NSO in 2023. It's interesting how there was a huge gap of just WL3 in 2023 and then suddenly everything you listed happening...