Most Hells are probably not paused. There was plenty of documentation - he wasn't paying close attention but he can call it to mind - suggesting that the plural is right, and also Karen wasn't in the place the Osirians describe, and not in a place with anyone else either, and while some worlds pause together - the Osirian gods seemingly having paused with their world, despite living in different ones - some obviously don't.
So you need a sprawling operation to pause all the Hells. You need to identify world after world after world and find people who've died and pull them back to the demiplane. The other Karen doesn't have the logistical resources to pull it off or the emotional resources to cope with it, because most of them will be much worse off than Karen was, most of them will want to stop existing, and because he's not totally sure the scope of it will be finite. So he shouldn't even mention it until the resources to accomplish it are at hand.
The helpful Valar, if they're really that, might constitute the resources to accomplish it - you could dump people on Lorien, give them a little time to figure out if they actually definitely for sure want to stop existing - and you could destroy his brother, at that point, with another one around -