You could still use those mounts on the frame. The supports will have to be angled to meet those lugs but they are ok once done. Most supports end up angled down to the mounts. Not heard about seat clamp mounts before but obviously a great idea. Nice bike too!!
Try looking at Topeak . I use their Rack and Rack bag and they do several attachments that can be used with their racks. Generally the fixing holes should be standard to take most light/reflector attachments whatever the rack you're using. I have an light on both the Rack and Bag, works handy. Check Evans too as they have very favourable returns terms should stuff not be suitable. I'd not be without a Saddlebag dead useful and tidy too.
Firstly, apologies for resurrecting an old thread! I'm currently looking into fitting a rack to my Defy 1, this thread has been very useful!
One thing I couldn't make out from my read through was: the seat post mount for the rack, which did you use?