How is one cured of the Filled With Life ability? Limited Wish? Miracle? Greater Restoration? True Resurrection? Reincarnation?
Hrmmm, aye, it'd probably have to be a Limited Wish spell level at least. Will adjust.

Also, since they're basically made of positive energy, would they take double damage from negative energy attacks?
Researched the few other creatures from the energy plane and it doesn't seem that any have weaknesses to the other. Probably because those from one plane just want to take, Take, TAKE! While the other is continuously giving, so neither are really affected by the other. Who knows. There is a void ooze which to my surprise is the same HD and everything as this fellow, thats from the negative energy plane. It doesn't have any special weaknesses, so its probably best not to give this fellow one.

Lastly, if one kills it, do those incorporated into it dissipate, or are they freed?
Well, if they have been absorbed they are now a single entity, so they'd both dissipate.