Well, now that the game's finished and on Ouya (yay^^) I now have to come back to those tricky and depressing questions about memory management...
First, thanks for the answer, second, you tell me that even if there is no obvious (do not mix with obviously no) link to a sprite sheet from any sprite on the stage, the texture will still won't be GCed after RemoveUnusedAssets? This is a rather disappointing conclusion if this is what you meant...
Thanks for this, I think I'll have to rethink my memory management all over (I cannot afford to completely unload my pages, or I'll have to rethink all the game structure)