OK, sounds like a good plan, if maybe unnecessary, but I guess it is better to be safer than sorry.
I have solved the first issue using this Action:
Format Value:
Field: Genre
Format String: $trim($regexp(%Genre%,'(?:(?<=/)|(?<=\A))\s*([^/])\s/(?=.?(?<=/)\s\1(?=/|\Z))',,1))
This will get rid of the duplicates in Genre.
I got it from here Merge duplicate words - #10 by dano
I really hope I can figure out how to propagate the specific genres from the Genre field (My second question) as I stupidly erased them after consolidating them in the main Genre field. I will play with abelcheung's suggestion and see if I can get something working.