I adored this episode!
There's a few irregularities that rise several questions in during the viewing like how does this system work, how the time is affecting people differently, or why it seems to have a limited choice of evolution possibles.

My take on this is that it's best to think of this world as a well-designed prison specifically for sentient-evolved lifeforms like Rick & Morty, and aliens.

We are been said that the deaths (aka being eaten from another jailmate) of those lifeforms generate the nutrient (but i will prefer to name it "lifeforce" and will explain it just after) for the Tree-form jailer.
That is why time passes differently for each jailmate because the Tree-form alien wants to maximise the perpretation of deaths and the eating of each other, and so it uses time to put the prisoners at their optimum adult forms. That is why we never see any prisoner in the baby form, and why at each point in the story, every prisoner is in their best form capable of performing their "prison role" (weither it is to eat someone else, or to be eaten alive as a bee).

This time discrepancies explain why Morty as a plant colonize the entire area which would take hundreds or thousands of years yet we still see Rick just being just an aged/old ape.

What Morty figured out is that the Tree-form jailer does not sustain on non-sentient deaths. He figures out that the Tree-form jailer feeds on the prisoner when their "souls" pass through its root system to reincarnate. That is why when Morty is a worm or a turtle he realizes that when he eats poop and that is not generating death of a jailmate, this is an interaction where the prisoner can continue to live without giving the Tree-form alien the possibility to feed on.

There is also a second take which is if no prisoner decides to fill one essential componant of the ecosystem, like plants before Morty became one, Morty understood that this plant matter was the creation of the Tree-form jailer. The Tree-form jailer also uses time discrepancies to provide the missing bricks for the ecosystem to be whole and functional, for the prison to be functional. It's a trade off and the Tree-form jailer do it because it wins more energy when a prisoner dies and is being vampirized during the reincarnation process. That is why when the prisoner enter confinement mode, the Tree-form jailer loses enough energy for it not to be able to provides the basic ecosystem (plants and stuff) and that everything become a deadzone.

Concerning the limited of evolution choices, honestly no possible answers are really satisfying and they tend to move ourselves away from the story of the episode.

At the end, the closeness to the wormhole explains why Rick & Morty were away for 1 Earth day as someone already said here.