1. Thanks 53. That barre on the third fret would only have to be across the top three strings, right?
2. I can see the practical advantages that the thumbed F has over a barre F, but not vice versa. Once I get this thumbed F to the point where I can play it reliably in context and not just in isolation with a lot of adjustments, why would I have to master the barre F too?
3. For the benefit of others who may be trying to learn the thumbed F, I'll explain what I was doing wrong, and how I learned to play it properly.
That two string barre on the first fret - because it's a barre, I automatically used the flat of my index finger (i.e., directly opposite to my nail) to press the strings. Then for my other fingers to reach their places, I was bending the first finger joint backwards - but I still couldn't reach with my other fingers, and that backwards joint flexibility doesn't seem to respond to exercise; conscientious practice wasn't getting me anywhere.
The breakthrough came when I tried using my index finger in a very different way - both first and second joints bent sharply in, and pressing the strings with the very tip of my finger - actually the B string is pressed with my nail. It's a delicate job to hold two strings firmly with just a fingertip, but it's doable, and unlike the flat-of-my-finger approach, it does give me the angle to use my ring and little finger.
I hope that was comprehensible and useful to somebody. I guess the moral here is: trying harder and practicing more will only help if you're doing it right.