So... could the bad keys be fixed in general? Are they rare enough to be worth keeping despite the defect? Thanks!

Beam spring has certain technical elements that make its feel a little comparable to a rubber dome, especially a very high quality one (i.e. Topre), but not even the highest quality rubber domes will match the beam spring switch in terms of tactility and the sharpness and cleanness of that tactility, because the "dome" on the beam spring is made of metal. It's a truly fantastic switch type, but the Model F does a good job approximating it. The primary difference, I contend, is the post-click travel feel; whereas the Model F's collapsed spring continues to resist travel noticeably, the collapsed beam spring simply collapses, and the resistance offered by the coil spring is quite minimal. It's next to impossible to avoid "bottom out" on beam springs, since the spring collapses so suddenly and completely.
A well-maintained Model F (as my allegedly new in box XT shows) is about as smooth as my beam spring, which can still get rough on deliberately bad presses, such as when I pushed the buttons completely horizontally. Centred presses, though, are as mentioned, extremely smooth, arguably smoother than Blue Alps. Build quality of beam springs is certainly first rate; however, at a given point, additional mass stops translating to quality; I like to refer to this as an instance of the law of diminishing return. My 3101 weights around 14 pounds, and I'd be just as happy if it weighted only 10. Being the single heaviest item on my desk, the IKEA desktop is literally bowing under its tremendous mass!