After the first few comments on glueing, I got a bit nauseated, so I didn't read the entire thread. Apologies if someone addressed this already:
The futility of glueing, bonding, welding, etc has been covered. But I want to add that any bike I've ever seen that was properly set up to use a clamp-on downtube shifter has some sort of tab or boss to keep the shifter clamp from moving down the tube. My Fuji has a brazed-on tab specifically for this reason. I converted to bar ends (which I very highly recommend), and use the original shifter tab to hold a similar clamp-on bracket where the brake cable housings terminate along the down tube.
Since your bike doesn't have this tab, I suspect any clamped-on shifter will gradually work its way down the tube, causing loose cables and damaging the paint on the frame. Or, aluminum being relatively soft, the clamp will dig into the aluminum, which will leave an usightly mark should you change your mind. Given that aluminum has no fatigue limit, I'm not sure I'd want gouges in the middle of an important structural member.
All of this seems to stem from your desire for better shifting. Why not just upgrade components (better brifters or bar-end shifters, for example) or get a different bike?