Why in this case is mhòr and not mòr?
Because adjectives are lenited after qualifiers such as 'gle', 'cho' 'ro' etc.
No they are not. They are lenited after specific qualifiers, such as glè and ro, but cho is not on this list.
She is definitely not saying glè, I've played it several times. I was undecided between "cho mòr" and "ro mhòr", never even considered "glè"