FTB Backups is one example, but unfortunately not available for 1.15.2
You're using an outdated version of Mekanism and possibly other mods, which probably had a bug which corrupted some stuff.
You should download the mods from CurseForge or from dev's own sites.
You also should keep the mods up-to-date.
In future regular backups of the worlds could be helpful, especially as the 1.15 mods aren't that stable yet.
Does it work if you launch vanilla minecraft (without forge)?
Also posting an F3 screenshot, some system stats (via lshw -sanitize) and maybe the system log (dmesg) could help us finding the issue.
Probably your chromebook doesn't have enough power to handle minecraft.
If you have access to the server, you can disable cheating it by modifying world's level.dat and on the player in playerdata/*.dat with a nbt editor, but the missed achievements won't appear afterwards AFAIK.
These not-rendered areas are often just not-loaded chunks which aren't covered by the far fog, which is very common and not a real issue.
How does it perform in vanilla (no mods/optifine/shaders)??
no mods?? more like a mod for the wrong mc version
BetterPvP_126.96.36.199_Forge_1.14.4.jar is for Minecraft 1.14.4. Pick the version of the mod compatible with your minecraft version.
The assets folder acts like a cache, you could theoretically trash the whole assets folder. then it would download everything again. (not recommended unless something is broken or disk space is low)
Unless you hoard skins, you can just trash it and call it a day.
The launcher actually should cleanup the skins afterwards, seems to be a bug somewhere
yes please send the full game log (any pastebin service, else in spoiler)
check if you're using the latest version of optifine.
else may try to tweak some settings like fast rendering etc.
you can contact mojang support. also are you running demo mode??
please send the game log and the launcher log
Yes, i support the playerCollisions gamerule, too!
You can post it to this gamerule thread: http://www.minecraftforum.net/forums/minecraft-discussion/suggestions/2486859-new-gamerules