As for the first point, it's a fair concern. However, there's already a solution on the market for that, active noise canceling microphones, in fact, my helmet has a hard mount for one built in. I've never used one myself, but from what I understand, they work rather well, though not perfectly, as the wind noise is rather, though not perfectly steady. Whether or not the remaining noise is enough to overcome the STT tech, I don't know, but I'm hoping not, with, I think, some reasonable justification, given the ability of google voice to comprehend me when I'm in an ambulance (which has a huge and noisy diesel engine) doing 90 MPH without a noise canceling headset. I'm reasonably sure that by spending a fair bit of time doing training, it will understand me tolerably well, at least over the crucial commends. Even if not, though, there's still use to it, if I can just set the google maps up before I start riding, and have it scroll any alerts across the top (which I believe I've seen "screenshots" of it doing, but I can't find right now), that will absolutely fill my need.
The second point, however, doesn't have much to say for itself. As it stands, I wear two different kinds of glasses while I ride. First are my corrective lenses, which I wear when my contacts are acting up, those just stay in place, between the ear hooks and the pressure that the helmet places on the side of my head, they don't move. The second, and more common, are a pair of cheap sunglasses, wince they have more or less straight bows, they do move some, but probably not enough to be irritating were they providing a HUD. Admittedly, though, the Glass is likely to be much heavier, and thus will move more...so it's probably fortuitous that I've already solved that problem by putting a couple of wedges/shims that are mounted to the faceplate (not visor) that comfortably hold them in place.
Here's the one you missed that's a much bigger concern than either of the points you raised...will Glass fit under my helmet without requiring modifications that will compromise the safety of the helmet? God I hope so, but there's really no way of telling until they hit the market and I can try both on at the same time.