I have a Horizon as well. The way I have have gotten out the lint out of the BACK of the tension disks, where I've had the problem is to cut a thin piece of 3x5 card stock so that I have a piece of card stock about 3/8" wide and 5" long. I then carefully put it behind the tension disks...where you can't really see, even with a flashlight. I then gently stick it down the back side of the tension disks and lift up and see if anything comes out. (Do this several times trying to find anything that might be jammed in there.) Two times I have had thread back in there...when I got it out, it sewed fine again.
I understand the frustration. Driving 2 hours one way is enough to make you want to try to figure it out yourself.
For me, just flossing the tension disks on the front side (as in regular threading) didn't do anything--the thread bits were where it was difficult to get to. Both times it was a little wad of thread stuck in there.
I have had things stuck in my machine as well even to where it made a terrible sound and I quickly turned it off and got the thing "unstuck" (and then cleaned it out thoroughly as you have). However, that was unrelated.