Alright I gave this a playthrough.
The dynamic of having to keep getting to low health is interesting. I love it! Having to leave and re-enter the room to get the higher multiplier which then stays until you leave the room is quite interesting too. Lets you collect resources en-masse at the right time, but then again you sometimes don't want to so you can easily drop your HP again, so that's neat.
Beat the super boss with max stats, 2k HP spare and 7 yellow keys, but I didn't take a green key for 1 swordsman hit in 7N.
A minor nitpick: The storage spot for the sword and shield in 4N is not all that useful, the place they would be useful is 8N as I ended up running back and forth a whole bunch just to use the wooden equips in 8N. Alternatively, don't block access to the shortcut by the orb at the top of the room and just make it immediately accessible once you make it to 8N.
As far as rewards behind the secret boss goes, it doesn't really matter. At the moment you optimise both scorepoints by killing one, loading and then killing the other. If the rewards behind the secret boss were worthwhile, this process would instead just be killing the secret boss then immediately killing the regular boss, and that's equally as uninteresting IMO. Both scorepoints are optimised via the same process in both cases.
The hold is pretty cool overall. I enjoyed it. A fun gimmick. Interesting that when optimising at the end you can only really take advantage of the health in 10N to the full extent as any other health caches all don't get to take advantage of the multiplier. Makes saving keys still work out despite many trades being like 1,000 HP + for a yellow in many cases.
Then again, the amount of HP you have for the bosses is basically just all the HP in 10N and HP saved elsewhere has little impact.