So, I tested yesterday and today 2 things, how the sequence is made and how the game treats input as I got into something very weird.
First, I knew that it has been "said" that you can't skip the pillar, however, the only info I had was so fuzzy that I had to check. That is to show the more the documents, the better.
Anyway, using the usefull peach warp in her first segment, I could check what happens to the pillar, they are basically all triggered and the doors are there, but I tested every pillar and they all would not allow you to activate them early EXCEPT the second one. That one, if you activate it before ending the ch1, it will play the cs of the pure hear (that you didn't get....computer logic :) ) and play the cs when the orange door appears. However, it spawns you back to where you were before (near when you would flip int he background) and not play the last cs right next to the pillar. The music also get screwed by simply not being there. In the end, the pillar actually never completed its activation because if you come back, you don't see the heart in the pillar and you can't activate it. The door actually never truly appeared so you would have softlocked. idk why this funky outcome happens,but it really seem that it just didn't completed the activation.
As for other stuff in the sequence....it's more solid than TTYD. Not only you cannot skip any ch access, but unlike TTYD, there's only one way to revert the sequence backward asnd it's pretty much useless. See, when you seqyuence break, you go forward, but by doing this, just like TTYD, it would make other triggers like the Pixls chest already activated. What this means is if you would skip a Pixl, you would not be able to get it back. In TTYD, there was some triggers that doesn't get activated by this and they prety much required you to trigger them by yourself, but doing so after a sequence break would revert the whole sequence to that trigger thus, allowing you to get another chance for the the triggers after.
This was good in TTYD, but spm only has one: the cat door of 3-4. For whatever reasons, almost nothing is activated in that ch except the locked door are unlocked.....not the cat one. The key chest are still closed so, you could pick them, open that cat door and it would revert the whole sequence to the Francis fight. In theory, this could be very usefull, but this means that to make use of this, you would skip 3-4 which is almost impossible because you need to end it to progress.
With the only possibility dead, it's tighter than TTYD because we can only sequence break forward and we cannot undo any sequence break so it will be hard to do it without skipping too important stuff.
However, I still maintain that the game has a lot of flaws elsewhere that can change a lot.
Now, the input....I wanted to test the life shroom gltich, but I got very inconsistent with the jump even with frame advance. I actually got that several actions like the Pixl and jumping requires you to hold down the input during a certain amount of frames. This doesn't apply to any characters actions which are simply a one frame input EXCEPT any jumps which requires at least 3 frames of holding to be processed after. I think I recall that Malleoz did came to that conclusion, but I couldn't remember exactly. Anyway, the amount of frames are explained in detail in this google doc:
https://docs.google.com/document/d/19HBFf3SuxsAjPzXSZFlPUSfwB_lpRsEWfsTiPQAya4c/edit?usp=sharing
Amd I have to say, this is the weirdest and most confusing input management I've ever seen. At least I know how to count frames for testing the input queue :)
So, that's all I had. The next part would be to test known glitch and try to draw some conclusions out of them.