Here, I have sorted your list according to my point of view. I rarely play SR, so I evaluated them all as seen in campaign. No idea how you could put Korvaak above Morgoneth.
That’s probably why.
To be fair, Morgoneth would probably be a real handful in high SR too, I just never met him.
Morgoneth (like Gargabol) is also disproportionately difficult because you can’t die and go back to finish him like you can with Korvaak, and if you die to Korvaak, you don’t have to start from the beginning either. But in isolation, he really hits like a freight train and takes forever to bring down.
Conversely, I’ve never had that much trouble with the mad queen, but that’s probably in part due to my personal preference for player-scaled pets and indirect attacks. Someone who prefers Melee combat will find her much harder. So she’s lower on my personal list than on yours.
In any case, this list is a consensus… If more people feel like Korvaak is easier than Morgoneth, I’ll adjust the list. 
Grava is surely harder than Theodin?