Zeality isn't necessary to explain the Pocket Dimension Theory; several of us could do it, and it was GrayLensman's theory in the first place, anyway. I'm not willing to explain it in detail right now, since you could probably find it by searching the forums (I believe the topic called "Four Lavos Questions" or some such title had the main discussion on this theory), but basically it's this: Lavos does not actually (or at least not necessarily) lie dormant in the planet for 65,001,999 years. Instead, he resides in a "pocket" of time and space called a Pocket Dimension. This connects through a wormhole (or in this case, a series/network of wormholes) to real time-space at several points: namely, all those time periods that you can travel to and find him in. He exists in the same form, age, and power at all these places at once, much like time passes for Crono and the party at the same rate in all time periods at once. So in reality, he could have been gathering energy through wormholes from the planet for a much shorter time. That blue vortex you always fight him in is the Pocket Dimension; you enter it, fight him, and come back out. I believe the only time Lavos actually leaves the Pocket Dimension is to rain destruction from the heavens in 1,999 A.D., and even then I may be mistaken.
I think I started to explain more than I meant to, and I started making less sense than I meant to. Excuse me. For full information, I'd suggest looking up any topics on Lavos that are still on the boards.