You should solve this at the plugin level. While this rewrite rule …
RewriteRule (.*)/?mysite.com/(.*) /$1/$2 [L,R=301]
RewriteRule ^(.*)THUMB /$1 [L,R=301]
… does what you are asking for, it is just cosmetic. Find the code that creates the wrong URL and fix that. Give the plugin author feedback; I’m sure s/he wants to know about that issue.