Great question and idea! But while I can find tensile and shear loads for 3/16" aluminum rivets on-line, a quick look didn't show any values for the aluminum machine screws, as it is alloy dependent. What I do recall from my GE machinist program training is that aluminum possesses the best strength-to-weight ratio of any metal in common use. It can even be made stronger than mild steel.
You mentioned the riv-nuts, I bought some and decided against it. Here's why ...
1) Per the Machinist's Handbook (let's say these are 'best practices') one needs at least 1.5 to 2 full diameters of threads of engagement for aluminum fastener joints. With the riv-nuts (at least the one I got) one would be lucky to get one full thread engagement.
2) Riv-nuts might be good for tensile loading, but I'm more worried about shear loading in an aluminum boat, as the pieces work against each other. And if the riv-nut was really strong - it could be too strong - causing the stresses to propagate somewhere else, like where it wasn't designed to go. Pictures of the typical load stresses attached.
I tell you, after re-riveting my boat back together with the solid rivets, compressed air tool and bucking bar, I am totally impressed with the rivet process! Dare I say ... it was even fun to do 8) !
In fact, I'd opine that if riv-nuts and bolts were such a superior method, why don't we see them used for structural methods in aircraft construction? I bet they're only used there for adding support items to the fuselage that might need to be removed at some point. But another point, they are more costly (2 parts to buy per joint) and more time intensive an installation, so that's probably the primary decision point for companies.