hydrocephaly
A twelve letter word for working well together is the word "cohesiveness".
"Alliteration" is a twelve letter word.
There isn't a twelve letter word, but if you add another letter "n" you get: entertain met (add letter "n") entertainment
factionalism
Librarian is a 9 letter word that has letters BRAIN in it.
considerably
overabundant
understating
environmental
chit chatting
unsuccessful
Appalachians