Why didn't Delenn use the Energy Transfer Device (from the episode Quality of Mercy) to restore more than 20 years of Jon Sheridan's life after Z'ha'dum? Membari are long-lived to begin with. 20 years from her to supplement the 20 given him by Lorien would have restored him to a normal human life span without even making a dent. So she lives for say 100 years instead of 120! She took a stabbing for him in Ceremonies of Light and Dark, indicating she'd give up her whole life for him, so why not a fraction of it? The machine was available well after the events of Z'ha'dum which we know because Marcus used it to restore Susan's life in "Rising Star." Also if Marcus knew of the machine's existence what are the odds that Delenn didn't? Even if there was some reason it would not have worked, it really bugs me this was not a plot point. Am I missing something?
Sheridan: I had Franklin do a complete medscan. He found things in my neurosystem. Some kind of biochemical energy. Repairing and sustaining me.
Lorien: I did the best I could. I gave him back a portion of his life, but only a portion.
Lorien: Twenty years. No more. And then, one day he will simply... stop.
Lorien's power here worked a miracle, restoring a dead body to perfect health. But it sounds very much like Lorien breathed his own energy into Sheridan. The Energy Transfer Machine doesn't seem to do the same thing. From The Quality of Mercy
That pain you're feeling, that agony... that's Lake's Syndrome.
We see throughout the episode that it's not strictly a one-way transfer. The doctor using it slowly takes on the maladies of the people she's "healing". In the end, the doctor transfers her illness to the man she kills.
There's no telling what Delenn or anyone else would have happen to them if they tried to use the device to extend Sheridan's life. It's also possible that Sheridan is now beyond anything the device could do for him. It might try to fix him, only to find him in perfect health (as the medscan did) and not be able to do anything. Indeed, when Sheridan dies he, more or less just stops living when he was healthy moments before.