OK, I got it fixed. For anyone else who would encounter the same, here are the photos and how I did it:
This is how tight the seatbelt was:
First, get a 19mm socket and a wrench with an extension and locate the bolt in the crack. You'll have to lean the seat a little bit forward and hold that pad up in order to access the bolt:
Undo the bolt and the seatbelt should come free:
Put everything back together and you're good to go.