I think that if the rumor is true that Sarah F will be invited to the wedding, it's an extremely shrewd move to keep control over her and her less savory commercial activities. She can't broadcast a play-by-play running commentary on the wedding if she's inside the venue as a guest, can she?
Inviting her is very, very clever. The BRF gets "credit" for being more human while continuing to ride herd on Sarah's exploitation of her former position.
Someone has gotten some smarts about the situation. It puts Prince William in a very positive light, while Prince Charles still gets to not-invite her to "his" festivities. Best of all, it keeps Sarah in line and under control.
If true, well played.