//Now I realized that I want to revise and book an open-jaw ticket instead (MNL - CDG, MAD - MNL) seeing that I'm currently confirmed on those flights already.

You might need to ticket them as two one-ways for 48K x2. Unless if you can see there are new MNL-CDG or MAD-MNL available.

//Also, in the first place, is it possible for me to cancel my initially confirmed MNL-MAD-MNL economy class flight and have them issue me the open jaw ticket (and disregard the rest of the confirmed flights)?

Yes