Lavos altering the genetic pattern of evolution is understandable, as that's his goal: to take the best DNA of the world and let the rest rot to pieces. He's like the Borg of the Chronoverse.
Wow, that's a horrible analogy, but you guys get that picture.
If we were to run with a mixture of the PD theory and a DNA theory, you'd only be able to get away with saying Lavos is in a PD so that he could muck with many different worlds at the same time (or, if you really want to mess with the entire thought process, to say that the PD can access multiple time lines). That'd be the only real reason I could see getting away with a PD, but unfortunately there's no grounding for it from in-game evidence. The way I feel it's described, the Lavos race lands on a planet, one per planet, draws the best DNA genetic material from the world, and then destroys it as a by-product of giving birth to Lavos spawns. These spawns then do the same thing: during their birth they are sent off to more planets, and they, in turn, continue the cycle of DNA manipulation, destroying the planet they are in, and then their spawns moving on. If that's not correct, then by all means, correct me please.
I can understand Lavos mimicking genetic material (heck, if it's a boss, the DNA must be good enough for Lavos to absorb) in the shell battle. But, my thought is how can Lavos mimic the Guardian if it's not genetic? Perhaps there is some genetic material, but the Guardian itself is a programmed machine. That'd be like saying Lavos absorbed the DNA of Robo.
Perhaps the people of the 1900s in the Chronoverse knew of Lavos (quite possibly by that time frame they knew what was going on), and had studied him to a certain extant. With all that knowledge, they could have tried to fight fire with fire, knowing that at some point Lavos would erupt from within the planet. So, they built multiple Guardians (each with the respective Bits) in an attempt to create an army which would be capable of defending against Lavos.
And as far as how Lavos mimics the Guardian, perhaps it's not so much an actual alteration like the game shows it to be, but perhaps it's a jog to the memory. Like, if I'm fighting Lavos and he suddenly starts using techniques from the Guardian or even Yakra, I'm probably not going to see a big flashy image of either show up out of nowhere and get "absorbed" by Lavos. I'll probably be fighting him, then he'll use Yakra Needle after maybe a flash of light for a second (not an image), and when he uses Yakra Needle, I'll be like "Oh, I remember Yakra doing that!". We don't get to see what characters are thinking, so maybe the visual effect that we view on our screens is nothing more than a visual representation of the characters going, "Ah, I remember <boss> doing that!"
And to make things a bit more interesting while answering the question of how Lavos can perform the same attacks...well, he is altering the DNA of the world through manipulation so as to get the best possible DNA over the years...chances are a bit of the world is Lavos...ian (don't know what the best terminology would be). Lavos probably infused his DNA to give the world a jump start.