Once you get above the tree tops the gusts are often stronger, but that's true at any altitude. I'm not sure that the tree line makes much difference for a drone flying well above ground level.
Correct, altitude and treeline has nothing to do with high wind frequency. The weather (barometer stability) is what determines the wind at any altitude. And like any place on earth, at any altitude, the weather can be hit or miss.
The Mt Lincoln video below was taken with flight altitudes up to ~14,300' MSL in Oct 2017. I launched from about 11,000' MSL. The craft flew fine, no abnormal behavior, but the wind was calm that week in the Rockies. The year before in Oct 2016 I wasn't so lucky, the wind was always about 25mph at all the passes. Hit or miss, same week of the year.
To answer the OP's question about RTH from high altitudes, if you follow the terrain of a mountain up to say 1500' (that's 1500' above launch point), but are sure that you are within 400' AGL, you are legal if you can
see the craft from home point. In most cases you can't see the craft if you're 1500' up a mountain. Everyone fudges on that FAA rule, flying FPV BVLOS, but in reality this is fairly safe IMO, as manned fixed wing aircraft won't be in that area, unless a helicopter comes into the area, which you have to listen for. You do have to consider the likelihood of other craft like ultra-lights, para-sailors, etc. However, if your craft goes into RTH for battery or disconnect reasons, it will return to you at the altitude you lost signal, at 1500' above home point. If you can't retake control to keep the craft within 400' AGL, you're taking a huge risk, because that's not following a key FAA safety guideline of 400' AGL max. The craft DOES NOT descend to your programmed RTH height before heading home, as previously suggested.
Today there is no way to know exactly what your AGL is when flying mountains like that. You have to trust YOUR experience at flying and judging the AGL based on your view in your device screen. Good judgement for AGL altitude takes some time to acquire using the screen view. When I first started flying DJI drones FPV 3yrs ago I would challenge myself to guess the AGL number based on my screen view, then look at the telemetry to see how close my guess was. After a while I could do a fairly good job at judging my AGL height within 100' when flying below 400' AGL. I typically fly at around 150 to 200' AGL, but in mountains more like 300'.....most the time.
Also, I deem the 400' rule to be within 400' of land, in any direction, just like a "structure". Logically that's a safe flying rule, even though I haven't seen clarity in writing from the FAA. However when I ask this question of the FAA at NAB a couple years ago, they said 400' in any direction of land is acceptable for AGL. This means when flying off a 2000' cliff, as was the case in the Supersition Mtn video below, it's OK if you descend within 400' horizontally of the mountain wall.
Someday ....DJI will include actual AGL vertical estimates, without range finding hardware. This is doable with cell Internet service and some software. By accessing Google's MSL database using LAT/LON coordinates, the app could be designed to calculate an craft's AGL for any location the craft flies, near real time (maybe one calculation every 5 seconds or so). The vertical AGL altitude could be calculated to about 75' accuracy (IMO), using: 1. The home point MSL from Google's database (using the GPS LAT/LON at take-off), 2. The craft's altitude from home point (generally accurate to about 50') using the craft barometer. 3. The craft's current LAT/LON position AND Google's ground MSL for that coordinate. I'm actually surprised DJI hasn't offered this yet. I seems so simple, compared to other things they're doing. Although it's not super accurate, it would be more accurate than a pilot trying to judge AGL from the display view.