Thank you for sharing your findings! My printer shows the same symptoms as yours.
So far, I have successfully followed your [https://de.ifixit.com/Anleitung/Canon+Pixma+MG3550+Disassembly/119426|dissassembly guide], and I have uncovered the mechanism that drives the finger that seems to block paper transport. I have moved the gears that I could reach with a spudger. I could not really make sense of them either.
Unfortunately, after reassembling the printer, the printer behaves just as before, and I have crumpled another sheet of paper. I am still undecided if I will disassemble the printer again, just as you did, hoping that it will fix the problem.
Thank you again for the excellent information! I would not have come this far myself, I am quite sure.