From what I have gathered your'e trying to work out whether "She said she can dance very well" is grammatically correct well in terms of colloquial i.e. in everday speech most definitely. Could and can have identical connotations both suggest one has the ability to do so but it is unclear whether they have shown that or not, the latter is contingent upon context.
I am not a teacher but I have been speaking the English language for long enough to understand its very fundamentals. Never the less if you feel you have not been adequately served yet than you may wait for another response but to me it is crystal clear at least from what I comprehended from your post. So in essence it could be could, it could be can. Haha get it... both will do since this isn't a flash in the pan.. Disregard the last sentence went overboard with rhyming.