Yes, the thread is very useful but unfortunately it's not aligned with the optical centre - it's offset about 16mm. My Image15 and Photo15 on the previous page show what I mean.
The magnification of the telescope is fixed at x3. It makes no difference how close or far the lenses are apart (within reason - from touching to a couple of inches). It may be that it's possible to cheat the system very slightly by offsetting the focus of the telescope and imager, but it's unlikely to be worthwhile.
Magnification is most strongly dependent on the focal length of the Therm-App lens. I've used it with 13mm, 19mm and (briefly) 35mm, all with success. It doesn't make sense to use the 6.8mm with an x3 extender because 6.8x3=20.4 - a very clumsy and heavy way to get essentially the same as the 19mm lens, but with less sensitivity.
The 35mm lens does appear to work rather well with the telescope. I estimate it makes the equivalent of a ~200mm lens on a full frame 35mm SLR. With a setup like that you can not only prove definitively whether bears do indeed *ahem* in the woods, but where, and how much. But if you're that interested in the habits of bears, either get therapy or a scholarship.