Its all in the numbers
You fiance needs to remind her of the reason why she's paying 1/5th - for the sat night. Back out the cost inc. for the premium night, how many people is she really paying for? That is her guest list. If 15 must go, then 15 must go.
RE: out of town guests. I thought the same thing - and every single one of them came! If you're inviting them, err on the side of caution that they might attend.
