"My question is about the support needed for the upper rack – is a single centered "I" support per crossbar, perhaps with steel plates cut at 45 degree angles from the vertical to horizontal bars for stability – enough structurally?  Or will I need two vertical supports in the middle, separated by 6-12” (or other distance).  If I can get by with a single vertical support, then I won't have to extend the width of the lower crossbars - but I want this thing to be solid. "

From my construction days, I worked with 2" square tubing stock on a regular basis. I think that if your going to gusset the cross piece as you mentioned, and weld it all together, it would easily hold the weight of two 60-70 lbs boats. IMHO what you need too consider and your welder friend/bridge inspector probably already knows, is that unlike the bottom cross rails anchored to the side rails, the 2 upright Tee's will get a "racking action" going both end to end and side to side. However you decide to fasten the bottom of the T's, you need to take that into consideration? If you want to be able to remove the upright racks, you could bolt a square plate to deck, with a big enough piece of square tubing welded in center that the upright 2" tubing will sleeve inside. Then you just drill a hole crosswise and bolt them together. Want to get fancy, you can use a clevis type pin for easy removal. You can gusset the bottom as well, that will really stiffen the rack up. I'd also consider welding a square channel between the uprights? Wouldn't have to be 2" square tubing. Just heavy enough to keep the tops from racking back and forth under the load.
 I'm with you, better OVERBUILT than UNDER-BUILT. I think as you start to "tack" the pieces together, any weakness's will show themselves. You want the WHOLE trailer moving when you shake the rack, not only the rack.