So you guys are volunteering to do the geometry calculations by hand? That's awesome. Too bad you both have terrible taste in calculators. HP FTW.
Making the linkage strong should be easy. Starting with bracing the triangles around the front of the tire. The hard part is doing it without adding 20 pounds of unsprung weight.
Progressive springs are the usual way for traditional twin-shock geometry, but my understanding is that a progressive geometry is better. Especially since the progressive action works on the damping as well. And yes, I've though of putting a pair of PDS shocks on this thing. If I knew how to tune them properly I'd go for it.
Version 1 will definitely be no-linkage, straight-rate, twin shock. For simplicity and expense. Depending on how well it works I'll try something else, but it's fun to think about the possibilities.