The Original Crystal could increase Kuja's lifespam, because it's the source of all life in the universe.
I'm just using a fact(the Crystal being the source of life) together with common sense(if the Crystal is the source of life, then it can give life or increase the lifespam of someone).
Yeah "he might have known about the powers and limitations of crystals", but what you have to remember, is that within FFIX's universe, the Original Crystal have no limits. It is the God of FFIX, the source of everything.Kuja knew a lot of stuff about magic, so it seems reasonable to assume that he might have known about the powers and limitations of crystals as well. I dont view 'Kuja not trying to use the crystal to increase his lifespan' as a sign that he lost the plot and went mad.