Me wonders how Frog understood the concept of DNA, him being someone from the middle ages.
I personally don't think there are multiple Lav- Lavo... Lavoses in the universe despite the fact Lavos reproduces. If so, each Lavoses would be capable of manipulating time, and time being a universal entity in the setting (if I'm not mistaken) time travellers would have detected them... I think.
Well, about Lavos' origin. Perhaps it's the final fate of the most advanced sentience in the universe (extra emphasis on "the most.") Being the most advanced sentience in the universe, they (or it) probably discovered a way to decode and integrate biological data of other living beings before anything else in the universe. They could create an omnipotent being with this, only if they could overcome one problem: lack of proper database. For this reason, they constructed Lavos (or transformed into Lavos) and started planet-jumping, collecting data and destroying them afterword, to prevent them from reaching the same point as the Lavos' creators has. During this process, the database has become so massive that the life form created by the integration could control time and space...
Or something. I think I lost myself somewhere.
Well, something to remember: The humanoid figure which the main characters fight is NOT Lavos. According to the final battle, Lavos core is the nipple thingy on the right of that figure. The figure probably is just a model of the most ideal life form created from the database...