Couple of ideas "spring" to mind.
I personally like the look of torsion springs but I'm not sure if they're any stiffer than most, might look better with that type of seat?
The other thing that occurrs is maybe fit a rubber buffer/stop in the middle of the spring on top of your slightly over long bolts so it bottoms out on them rather than the mudguard. Along the lines of a rubber door stop.
My seat springs (lycette type seat) attach slightly further back than yours on the end of a short steel bracket which attaches to the same place you have yours bolted on. (just a bit of flatbar with two holes in it). The thing I notice is it makes my springs angled slightly more backwards, I'm not sure if this would solve the functional problem but might be a little better aesthetically.
