I think همـدلـی and همبستگی are better equivalent for solidarity!
Hello again Faylasoof,
"یک جا شدن " can not be found or heard in daily conversation in Iran. It is the literary form of "come together" , "join" , "unify". I like this word because it's a good poetic word.
PS your synonyms are great. I like hambastegi too.
TC