I didn't mean to imply you weren't doing enough. I only got from your post enough to reply the way I did. Sorry to imply otherwise, if I did.

If you want to know if it is the cable, then completely disengage it from the rear derailleur, pull the housing out of their respective stops, and slide the house along the cable to find where it is binding. Lube it as much as you can along all sections where it sits inside the h ousing and then wipe it down before reattaching it at the rear derailleur. You will have to readjust the cable tension if you decide to do this.

Check both ends of each piece of housing, especially where the ferrules are, and double check that there are no sharp bends or kinking that could be binding the cable, especially the piece coming from the brifter that you suggested is too short. I know you said you did this, I am just trying to cover all bases.

It sounds like corrosion to me, except corrosion is more common "south" than "north." It could be binding. It could also be your rear derailleur hanging up (lube it at all pivots just to cover that base), but it doesn't sound like it from what you are saying. Honestly, it could be a lot of things, but I am just trying to lend a hand via a forum where I can't see the bike.