Well, I have said before that I do think Felder brought something to the live performances that is lacking now. Sadly, I never got to see him live, but in all of the video I've watched (which, yes, is a LOT), the way he and Joe played off one another was something special to watch. Steuart's stage presence is much more low key, but in fairness, that may be intentional since he is not an official Eagle.
I don't consider the band all Frey and Henley either, although I think it's safe to say the band would cease to exist without either of them. However, the analogy I've used before is that the Eagles are like a cake recipe. Some ingredients are more important than others, but if you leave anything out, you can tell the difference. And some ingredients can be substituted for others without much degradation in quality, but not all of them. At this point in the band's history, if is doubtful that any of the remaining four members would be replaced.
Regarding if Felder could have been as successful without the Eagles, I can only go by what he's done before and since he left the band, and, of course, the answer is no. But, again, in trying to be fair, I think that is true of all the band members. Even though Henley, Frey, and Walsh all were pretty successful on their own, they didn't have the success they had in the Eagles, which goes back the point that it's the combination of their talents that makes the Eagles so special.
As far as Frey and Henley tossing good songs that the others wrote, I can only say, look at their track record. Maybe the Eagles wouldn't have been as special if they had recorded those discarded songs instead of the ones that they did. It's hard for me to argue with their success.