There were a lot of women in grey dresses at the wedding of Louis and Tessy because grey/silver was the color of the wedding. the closest I could find to your description in the photos I have was Archduchess Marie-Christine who is, as we know, his cousin.
I think that Felix is tall,I don't know how tall but...in some pictures it appeared that Felix is not so taller than Guillaume,but in the others it seems different!
I found a blog about the Grand Ducal Family.
And about Felix' love life, there was posted a comment on February, the 18th, 2010:
Just letting all my fellow Lux lovers know that Fel-icious is in a serious relationship and has been for some time. I've been about 95% sure of this for a while now. I've finally had it confirmed from an irrefutable source that the information I had was accurate. Our dear Felix is unavailable.
Lëtzebuerg: Prince Felix