My guess is that the Eye fusing with the MC functions similarly to forced evolution from the serum. The level 5s who evolved from the serum are implied to be weaker than the natural level 5s because they didn't cultivate as strong a foundation of power for the evolution to build upon. Having the Eye push the MC's body straight to level 5 from level 2 or 3 would result in an even weaker product, because the foundation would be far lesser. Also, the Eye's own strength is dependent on it reuniting its fragments, and it relies on the MC's power for that. So the weaker the MC is when it fuses with him, the more unsteady the foundation and the less power it has to augment it with. I'd expect, then, that a level 5 MC would be stronger than the Eye fusions we've seen, but that an Eye fusion built from a level 5 MC would be the godlike being the Eye is supposed to be.
In fact, natural evolution isnt better, someone able to evolve naturaly is just better than another who cant, and if that someone who can evolve naturaly were to use the serum there would be only benefit from it, it was explained by Weirdworld.
The natural one take a tole on the superhuman body and is far more risky.
As for the eye during is fusion with mc, a very clear indicator of his true power is how Bernhardt react to him and take him very seriously unlike Eisheth, who's a chosen and more powerfull than most level 5 ( And he had already met Eisheth and seen her full power at that time)
Can Liz still hax enemies to the Great Ocean, or is that just a monster world only feature? Maybe a later evo Liz can teleport all the way there, but I don't think she'll have access to that kind of instant win button outside of the monster world.
obviously not (at least for now), even Nico cant go that far and she level 5
amber and liz are level 3 as they evolved when they woke up with their powers and it's stated that twins only evolve twice because they jump from 1 to 3 to 5 so its less they need to catch up and more they need actual skill and experience to hasten their evolution as well as see if they can fix or help the situation with their body.
Amber and Liz arent level 3, they are two superhuman in one ( even if Amber is a special case)
When they woke up they were level 1, then they became a chimera thanks to Tanos so they share their pool of power in one body.
To compare , Devana and Evander who are twin became superhuman (level 1) then Tanos prepared them through some training and then he induced an evolution artificialy in both of them at the same time to reach level 3.