The pipe in the switch palace just leads me back to the start, and there's nowhere else I can go. Is there something I'm missing or is it really that much untested?

I've played it, but it's gotten me confused. Why do two of your levels not have exits, one being the Switch Palace which is needed to continue playing?
Also are you aware that the last jump at the end of the second level is impossible without taking a hit, as well as the jump at the end of the bullet bills, so in order to beat the level you have to bring two mushrooms from the start of the hack?

The first room of level 104 appears to be impossible (as the solid block to stand on turns into a coin the moment you touch the POW to make the door visible). Also for me the magikoopa's stationary magic in level 103 wouldn't spawn 16 out of 16 times I tried, making the level impossible.

...I doubt that. You say you managed to enter that door, while the invisible coin blocks are in your way? The only way to get high enough to enter the door is by pressing the P-switch beforehand, making it impossible to use the P-switch to enter the door. A possible solution to that section would involve duplicating the POW block, but I really don't feel like doing that. I know that it's quite easy to do a P-jump, but even with a P-door-entry that section is not possible (without using block duplication, that is).
So to repeat myself: did you really manage to beat that section?

As for testing the other starting areas of levels: the Magikoopa's Stationary Magic still won't spawn for me. Rather than adding jumps please fix this first.

Oh and I completely forgot to mention: don't be so generous with capes. I flew over one or two levels and kept a cape to the impossible level (103) probably allowing me to skip a lot more. I'd highly recommend removing all cape-based traps from your hack, not doing so will result in me skipping nearly all your levels.