Merlin doesn’t really lower (or dump) mana, he makes the affected hero use the special on a teammate once mana bar is full. When you’ve cast him, you want to send as many tiles as possible on the hero under his spell.
Hansel and Gretels specials dump mana - the affected heroes mana will be cut in half and it will take extra damage when mana bar is full. Again, when you’ve used them you want to send tiles on the target to get the most out of the special.
Proteus and Hel locks mana bar where it is; but if the mana bar is full and hero is ready to go off when you cast them, the affected hero will still use its special on you.
I have and daily use all of the above and find it hard to rank them, as they are all fantastic in their own way. Proteus/Hel play well with the others as well as with each other. Last 4* tourney (no red bloody battle I think) I ran Proteus, Gretel, Jackal, Hansel and Caedmon and it was the most fun I’ve had since I started this game.
However, I agree that Proteus has priority over Merlin. The latter might prove extra useful come November though, if Neith starts showing up on def teams all over (I’m holding on to some emblems just in case).