Well, we finally figured out that the IPS patch caused the bug!
Once you arrive to the castle level, an instant death happens as soon as the level appears, not letting you the time to do anything.
When we took the patch BOBO's made and tried to apply it on a SMW American ROM, we discovered that some level entrances were inexplicably moved.
But, if you download the ROM version of his hack (are links to ROM files allowed here? I don't have the time to read the rules, sorry), this problem is absent. I'm recalling the link which was posted a few posts above: [Removed by myself for obvious reasons]
EDIT: I'm just realizing now
what people meant with the second slope in the second level (keep in mind I'm not the author of this hack, only an assistant in its making). This is because it requires an awful lot of timing to achieve the right jump at the right moment. BOBO is just doing that so naturally that he didn't realize it when designing this part of the level.
Here is the exact place where he jumps on his footage:
Of course, being a true Mario sucker myself, I'm hardly able to reproduce that even by juming at the exact same moment. I didn't mention that you had to slide down the slope first in order to succeed (edit: lol, I finally did it, you don't have to press right when you jump, but you have to maintain the accelerator pushed on when sliding).