well if he had his body back couldn't he just use the Escafil device and get powers himself. Its not like it wont work anymore. maybe that is why he had it. he wanted to be human again or something Who knows
Quite a good thought as for why he had the device, but as for the not morphing to save his own life I really think it's a KASU. Even if the Ellimist didn't give him morphing power when he got back he had plenty of time to regain it, and saying he doesnt have the energy in TAC instead of simply saying he isn't capable implies he is capable. As for him being too tired? I've never bought that.
The first book contains a few things that dont hold true later, such as human thought speak, and Tobias having knowledge of the yeerks that he just needs to sort through and, indeed, morphing failing to heal wounds. Morphing didn't fix Tobias' split finger (I tell myself he got scratched after he demorphed but why the heck would the cat do that? Nevermind he kind of
states it happened before he finished morphing) I can see K.A. letting Elfangor die because morphing to heal his injuries wasn't actually possible at the time, thus she claims he was just too tired in TAC, by which time she'd decided that morphing would heal injuries.
To me (and mind you're free to disagree) being too tired is an excuse and nothing more, it doesnt hold up under scrutiny, it just doesnt, you can't even try to say that humans are built of sterner stuff because we see Ax morph to avoid death too, we see the Animorphs on the verge of death still manage to morph, sometimes they're much closer to it than Elfangor was.
Elfangor can strut around, handing out morphing power and taking swings at Visser Three but he can't morph to, quite literally, save his own life? I don't see him being so weary with war that he just gave up, not right after meeting his son. There was still time to morph at that point, even though he'd been spotted he still could have morphed to his bird morph and booked it out of there, the construction sight would have given him places to hide from the yeerks who couldn't go around draconing the whole lot without drawing attention, and him escaping could have allowed him to lead the yeerks away from the Animorphs and provide a vital aid to them through the course of the story. I think K.A. just didn't think that morphing would heal at the time so Elfangor didn't even try it.