The upwards acceleration model does seem to be the "official" position of the FES. I have seen some others (on YouTube) saying it is simply density, however if you dig any deeper into that statement, it becomes clear that density still needs a force to move anything, and that force comes from gravity (whatever you think gravity might be). So I'd have to agree that the "Universal Acceleration" model is at least better than, "it's just density".
As so often happens, if you dig into the upwards acceleration thing, you're going to run into nasty problems. Probably the most obvious is the well-known and often repeated Cavendish experiment. I have seen FEs point to all kinds of perceived problems with that experiment and attempts at alternative explanations, but it always comes down to a simple basic agreement. The Cavendish experiment shows that matter attracts other matter at least to some extent.
The next nasty thing you'll bump into is that gravity varies on the Earth. If you climb to high altitudes, gravity drops. If you go to the equator, gravity drops. These I have seen "explained" via the concept called "Celestial Gravitation."