Safari Extensions keep on getting deleted
Most likely: Another case of self-corrupting but never correcting caches. 


Close Safari.
Remove ~/Library/Caches/com.apple.Safari (the entire folder  and its contents, where mismatching copies of the extension pile up with a pattern of name-1.safariextension, name-2.safariextension .)
Restart Safari. 
Download the extensions you need once more.
Extensions should stay put again.