I wouldn't be surprised if William didn't attend, and attended the FA cup instead. It was one thing to miss his brother's wedding reception, but a distant cousin's wedding? We know he has missed his cousin's weddings before. He attended a friend's wedding instead of Peter's wedding back in the day.
Considering the wedding is at Windsor, I'd expect the royals who are able to (no conflicting schedule) to attend. I would be surprised if the queen didn't make it since it is at Windsor.
Freddie's wedding is no sign of what may or may not be. Freddie had a more private wedding, and a much smaller guest list. It was an intimate private wedding. We don't know if he invited any royals, to say they didn't attend.
I'd be surprised if there weren't a few Greek royals in attendance. King Constantine is her godfather. Her godparents include Constantine, Marina Oglivy and Prince Mariano Hugo, Prince of Windisch-Graetz. Mariano's wife is second cousins of the Kents (her maternal grandmother was Princess Elizabeth, the elder sister of Princess Marina).
If she has child wedding party I'd be shocked if Maude was not her Aunt's bridesmaid. Possibly Isabella but she is only 3. I know we have seen that age in royal weddings before, but depends on if the child can handle it. The Duchess of Wellington is also one of her godparents. Perhaps one of the Wellesley grandkids?