1) Oculus Go overheating issue:
You could adjust the "FFR level" in Global Setting (when you in the main hall with mountains outside, look up and a hidden bar will appear, along with a gear icon for "Global Setting").
Change the "FFR level" to Medium or High would help alleviate the overheating issue.
However, with the capability of Oculus Go, you will definitely have overheating problem from time to time (assuming you are watching high resolution videos).
2) Well I would definitely suggest to put max. 50 videos in one folder.
SKYBOX needs more time to go through large amount of files and probably will freeze when you put 250 files in one folder.
3) 60Hz is an optimal choice when you watching videos with 30/60 frame rate.
The default refresh rate is 72Hz on Go, which is optimal to watch 24fps videos.
4) No there is no such way.
Hope this helps