CA Glue
I’d just made a note for myself to try flexible CA glue on my next set of bristles! I have some of this around, and will give it a try:
I hesitated the last time because I’d run a bit of beeswax on the taws to keep them together while pitching the body of the thread. I’m guessing adhesive should come before any lubricant.
Heat-shrink tube is a thought. I have experience with wider diameters for electrical wiring and other projects, but haven’t used any of the really thin stuff. I do wonder whether you’d get a lip on the leading edge that could snag or pull apart. But there’s no telling 'til someone tries.
If you can find really narrow tubing, it might even make sense to just shrink half a length over the end of the thread and shrink the other half on itself, to use as a bristle.
Heat Guns
I do have a heat gun, as well as a hairdryer. Sometimes I’ll use the heat gun to speed things along with contact cement when bottoming in the garage.
As it is, I end up working a lot in my apartment, at night. It’s a bit noisy, so the heat gun isn’t the first tool I reach for. Thanks for encouraging me to give it more thought!
Tongues
I ended up fitting tongue over vamp, just as you said, in my redo of this pair. I have photos of the process starting here.
I’m about to last those uppers, so time is still to tell how I did. Patterning-wise, I chickened out a bit by intentionally cutting the tongues too tall, essentially giving trim allowance.
You crepe-sole low boots look great!