Oh wow, you responded quickly this time!
Anyway, about the teleporter level, I think the easiest band-aid solution would be to dig a hole in the ground before the finish line and put a new IN teleporter in there that leads back to the third painting's room. I went back to check, and all other rooms have their own IN teleporters that (eventually) lead to previous rooms, effectively allowing the backtracking I'm asking for (even if unintentionally sometimes); the only exception is the goal room. If you're feeling generous, you could maybe even add an extra room in the top-right of the level for the teleporter to stop by first before sending the player back to the third painting's room; that way, players who already have the paintings but are curious will get a small bonus instead of just being sent back.
By the way, since the Snow Queen abruptly summons hazard snow around her after she finishes teleporting, I think the teleportation-effect snow should be widened to cover that entire attack area instead of just the spot where the boss is spawning. Maybe you could even have some flashing bomb-snow appear there that's harmless at first but "explodes" when the hazard-snow appears.
Oh, and maybe have some warning for the boss's stalagmites, too.
Also, I know you said you don't want to overdo it with the stronger powerups, but I gotta admit, I'm a little disappointed that each boss room doesn't give you a different one; the Snow Queen just gives you the same whip as Patient Zero. Like, not even the long whip instead? I don't think you'd have to worry about the long whip reaching past the hazard snow, so it wouldn't make the fight that much easier (it may even mean the stalagmites don't need a warning since you'd be far away enough that the first one or two would be the warning), but oh well. I wouldn't mind if you didn't take this suggestion.
(edit: at least the long whip wouldn't make the fight be nearly as easy as the Reinforcements' flame cannon makes that fight; one charged flame drained basically all of its health in seconds. In fact, I suspect that this world as a whole is easier than the previous two; this might need to be the new world 4 and Caldera Keep moved to world 6. Admittedly though, my main issue with Caldera Keep was the blind jumps that you may or may not fix later, whose absence might put it back down in difficulty.)
EDIT: I was thinking about the inventory mechanic some more, and I realized: because you can only use an inventory item in the overworld, it's effectively not much different than just going back to a Reinforcement. This suggestion might require too much overhauling for this game, but for the sequel, I recommend letting the player use inventory items within the levels, but counterbalance this by having fewer powerups throughout the game in the first place. That way, there's more of a risk/reward system because if you keep dying on a hard segment within a level, you can try to use one of your now-much-more-limited inventory items to help you out at the cost of maybe not having it later when you might need it more. Plus, it would justify stuff like the original Dismemberment Ward's second painting because it would be rewarding players for keeping the right item in stock rather than punishing players that didn't go in with the right item equipped in the first place. Oh, and it would provide more incentive to buy powerups from the store, something I almost forgot was a thing.