I thought you beat LoH:IU a long, long time ago Dimi?
Anyway to answer your queries. The quest is definitely intended as beatable. Maybe there's a bug with 2.55 or the later versions of 2.53. LOH:IU has been painstakingly tested in one of the earlier versions of 2.53, but its not the final one. To prove that everything is possible I made a playlist on youtube. See: https://youtu.be/Yjkgi5GSURI
Care to show me a video or screenshots of the room that you say won't let you win? As far as I remember the only room with a triforce and a jinx cure that won't let you go through the left door is fortress 7. And its intentional that you can't got through the left door on that screen, but you can still pick up the triforce if you know a neat trick. You were never meant to go backwards with an item cure in fortress 7.
Of course things are rehashed from the original. What would you expect when the title is the same except for the insanity unchained part of it? Its supposed to be an improvement on the original game - namely scripting which was absent in the original.
The door leading of the map in fortress 5 is just a decoration. Don't worry about it.
Fortress 7 is supposed to be the hardest fighting sequence in the game - even harder than fortress 9. Hence the difficulty is in the shop. Its designed to mimic the world of business which is mean, unforgiving and brutal. Each enemy is a customer and they will pay you money when you kill them and collect the rupees. You use these rupees to pay your supplier for jinx cures, bombs and health. If your income is greater than your expenditure, then your business is profitable and you will eventually be rewarded with the triforce. In such a scenario, you need to constantly serve your customers and spend very little on your supplies. Are you a good enough soldier to manage it? Too many mistakes and you'll go bankrupt and have to restart.
The big wallet is still useful even when the limit is set to 500. Simply because if you're doing well, you can build up a surplus instead of being capped at 500. And this surplus can be used if/when you stumble and make a few mistakes. And yeah, the balance has been finely tuned including the shop prices. I don't want to destroy it and I don't want to make a lite mide.
In Fortress 6, you are supposed to reach that gohma room with enough rupees to spare. That means you can't use the lens too much. You should only be using the lens when absolutely necessary. Once again, I like to play my game brutally hard and so the balance has been designed that way.
The issue of difficulty balance is something that's not really possible for me to fix given that this is a free game. I make my difficulty balance to suit my tastes and I have absolutely no idea what anyone else's difficulty tastes are. The only way I can find out what someone else's difficulty tastes are is if they extensively playtest the game with me. And I don't think anyone will extensively playtest my game so its not really possible to design a game to please others. The best I can do for everyone else is to make the game with no password so people can go in and fine tune the difficulty to suit their own preferences.
Isle of Rebirth wasn't designed to be played with 3 hearts I think. Anyone can artificially make a game hard by putting artificial restrictions but will it be a "fun" hard? Will it still be possible? LoH:IU is impossible with 3 hearts. I already know one room you'd never beat in a million years with 3 hearts.
Anyway, well done on beating LoH:IU. I offer you my congratulations.