no it won't (see below)
yes, all culture slots are occupied, except that there's the mesoaemircan culture which is not used for anything
and religion slots are not full. i added a new religion, and i don't know how many more can be added, but the point is they're not all ocupied
Nurn that would be
they're not same culture (again see below)
you are only partially right.
culture = building/interface style, portrait sets, building/event descriptions etc
cultures are Orc, Dwarves, Elves, Gondor, Men, Evil Men (eastern_european, southern_european, greek, gondor, northern_european, middle_eastern)
religion = the thing you need to recruit troops

- in the game it's also displayed as 'culture' which causes confusion, but the two things are separate.
religions are Elves, Dwarves, Followers of Melkor, Dunedain, Northmen, Others (orthodox, islam, catholic, numenorean, heretic)
so isengard could be made a separate
culture without screwing the crusades (invasions)