I like the fact that this class can become stronger by slaying creatures and 'taking their strength'. I would decrease the amount of kills needed because hardly anyone would ever get 1 actual ability point in it before dying. Maybe only have it work on creatures who are at least the same level (or ECL) as you are? (to avoid any 'slay village, acquire strength' moments).

Cheat death can be 'abused' with an infinite mental ability scores loop if the campaign actually takes years.
=> One day before middle age become lvl 17
=> day later become middle age, recieve age bonuses and penalties
=> Cheat Death, no more physical penalties
=> rinse and repeat every year.
Generally campaigns don't last years in-game, so I guess it is ok.

Is the limit of 2x level maximum per round on Mana Use still valid on Cheat Death. If yes, the class feature is useless before level 50. And even then it would be generally not needed anymore.