...How did Elfangor, Arbron, and Alloran manage to keep control when they morphed Taxxon for the first time? Tobias and Ax lost it when they morphed one, and they made it seem so difficult to control a Taxxon mind. Why was it so easy for Elfangor?
Well, it certainly wasn't easy, and Arbron didn't entirely keep control of the morph... Elfangor had a time of it too, although he gained control at the last second.
I think a lot of it has to do with mental discipline. Alloran is a trained warrior. Arbron and Elfangor are warriors in training, and Elfangor at least is a lot more serious about training than Ax ever was. That leads me to believe that they have a stronger ability to quell the natural instinct of the morph than Tobias and Ax.
I don't know, I think that it is still somewhat of a plot-hole/discontinuity that it at least seemed more difficult for Tobias and Ax than the earlier trio. The others may have had a good amount of mental discipline as a result of their training, but Arbron and Elfangor can't have been much (if at all) farther along in their training than Ax: It was regarded as fairly unconventional for their Captain Prince to have sent two
arisths so young even on what was expected to be a routine, simple mission. And Ax may have been one to daydream during classroom lessons, but he is almost comically devoted to the warrior ideal and took his training and discipline as a warrior at least as seriously as Elfangor in an effort to live up to him. And even with relatively good general mental discipline, the Anis are vastly beyond Andalite warriors in morphing experience and practice controlling a morphed-mind (The only Andalites who might approach the Anis in morphing experience would be
estreen who make a career of morph performances).
I suppose there are some outside possibilities like that Earth-meat happens to smell much more powerfully attractive to the Taxxon senses and brain than other Taxxons and the other stuff that happened to be around on the Taxxon home-world; though those aren't the kind of 'single possible answer' that this thread is looking for, so I think that question is still kinda open as far as canon confirmation is concerned.
Another thing that crossed my mind (wow I ask a lot 
)
About Mertil, if I remember correctly we never actually know why he can't morph. I think it was Ax who guessed that he was maybe allergic or something, but is it ever confirmed?
I don't think it is ever confirmed in the sense that someone who actually knows stated that it was the case; but I still think that it is a safe assumption that that is the simple, single answer since it is the only thing that immediately occurs to Ax as a reasonable explanation. Still, even the term 'allergy' may not actually mean a literal immune hypersensitivity disorder, Ax isn't a scientist and the word may be used colloquially by average Andalites to describe an inability to gain the ability to morph that may be some other reason than what is actually an allergic reaction.
