I was able to open it up and fix it. I also made use of this thread:
https://www.eevblog.com/forum/thermal-imaging/i-need-the-blocking-pin-for-seek-thermal-compact-pro/. Mercifully, the threads were not stripped, just skipped.
However, I didn't find any of the explanations of how to do it very clear, so here is me spoonfeeding anyone who has this problem in the future:
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The lens is threaded into the camera body, somewhat like the common cheap webcams, and it is this thread that does the focusing. The knob-like housing is mostly just an actuator to move it (and a housing to protect it). It is clipped into a ridge on the camera body with three fairly small clips, and there is also a pin that limits its rotation to a single revolution.
The lens has a head with splines on it, which engage with internal spline-like fins on the ID of the knob, and one of these fins is extended and engages with the pin to limit the rotation.
To fix this problem:
1. Do not disassemble the body of the camera.
2. Get a very small thin screwdriver, and very carefully pry the bottom edge of the knob
outward radially,NOT upward. You'll find the point where it comes un-snapped, and then move 120 degrees and pry at the next location, and it will very clearly start coming off, and then you release the next point at another 120 degrees and it comes off.
3. Now you see the screw-threaded lens itself. You can operate the camera, and set the focus anywhere from worse-than-infinity (screwed pretty far in) to infinity to close-up (screwed out) to super-macro-short-range (screwed even further out). Probably you want the camera to focus from just past infinity to as macro as possible within one rotation.
4. Set the camera to focus at infinity and then rotate about 10 degrees past that (screwing in) so it's past infinity.
5. Find the taller fin on the inside of the knob, and use a pen to mark the outside of the knob.
6. Carefully, without rotating the lens more than one notch, snap the knob back onto the camera such that the long fin is counterclockwise of the pin -- i.e. so that you can unscrew the lens one turn, and screw it in barely any.
I got it working again, and as far as I can tell the FFC problem was not real. It does however not focus as smoothly -- I think I marked up the focus bearing. Ah, the joys of very cheap hardware.