Well, I'm not an emulator writer, and I think you mentioned it before somewhere else;
I looked at the border code, and it seems that you exploit that there are no 'switches', but rather being at the desired 50/60/70 at appropriate positions that counts. Instead of HI/LO at the left and 60/50 at the right, you do HI/50/LO and 60, respectively. Are you in HI for four cycles longer than "usual", and is it this that frees you from needing a stabilizer? If not, why don't you need a stabilizer?
For the multiple scrollers, I haven't looked at the code, but I was speculating that you use several sync scrolling parts here. Is that correct? Kind of "folding" the screen memory to scroll the lines individually.
I was not at STNICCC, but I was still confused the old school way when I saw it.
It's super fun to try to guess or figure out how you do the stuff.