I hope that the rumours are true (but one never knows before there's an actual announcement). I think the couple have been seen attending a wedding together in December. Some time before that they attended some gala together. So there have been signs that they still are a couple, and that it might be rather serious.
As for a wedding tiara: My guess is that it'll be the Aosta Tiara which Princess Astrid always wears. How I'd like to see that beauty worn by a bride !!!