Whats the shortest stem I could run and still mount my Garmin 800 on it?

Managed to get mime on a Thomson 70mm no probs

Not sure

You can get a bracket / mount thingy that clamps onto the bars and then holds the edge in front of your stem. Looks quite neat but not sure if it would be suitable for mtb use with the added crashing potential!

If you’ve got no spacers above the stem then you can go pretty short. I have a 10mm spacer above my stem on my road bike (100mm) and it is tricky to get in place. Solved easy enough by a foam “shim” under the mount.

I’ve also seen a fair few people run them on the top tube if they are just recording as it has more space and might minimise crash damage.