indeed mkdir : works fine anywhere. but can't add files in it if it's in the shared storage! 
but my question there persists for those files which didn't have : or | in them, such as the same old TXT.rtf. those still work fine, except during the big copy/sync. (although i haven't tested | nor do i know if other possibly broken special chars)
in either case, i think we both can agree the main bug was in android after all!
Yes, definitely some android oddity that I haven't seen documented anywhere! The recommendation I've read says only use Windows compatible file names as external storage can still be on FAT32.
Maybe invalid characters in the directory names?
nope.
there are certainly "special" characters in them, though.
achoz is a symlink to storage/shared/achoz.
you could probably reproduce this on your side by creating data/data/com.termux/files/home/achoz/.nosync/toreview2020dec/(∆16G)to organize/(∆3.7G)mac+documents/zona/BlahTherapy - May "premier plane".rtfd/TXT.rtf on google drive
as stated before, this file can be created by termux easily, and even by rclone copy if pointed directly to that folder.
but it shows as that error from the very first original post in here when copying the whole structure.
at one point, i was betting it could have something to do with the double quote mark ", because i had that minor issue with using it instead of single quote mark to create the file using touch, echo, or whatever.
but i'll refrain from digging this further.
i'm really happy with the solution and i'm already moving forward with my original quest. 
thanks again for all your help! 
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