Man Of Two Worlds by Frank and Brian Herbert
Edit to expand on this answer
I too was trying to remember this book for years. After reading this post i carried on using google, remembering a few extra points, like how the eyes swivelled inwards, when they are creating things with their minds. Google lead me to the possibility it could be Man Of Two Worlds, and it kind of fitted, since i remember reading it around the time i first read the Dune series. So I bought it, and it is the book we both remembered :)
Points you remembered . . .
The aliens in the book are non-humanoid, like blobs.
Blobs with 4 legs, 2 arms and "floppy" ears
The alien species can create things with their minds
By "rotating their eyes inwards" they can create living and non-living matter
They may have been responsible for creating galaxies, including ours.
One of them created our galaxy and is referenced to, but not in the book directly.
There's a scene where two of them create their offspring by willing it into being simultaneously.
Kind of. They use a "seed" and use their minds to help grow it, and pass on traits they wish their offspring to have.
There's an alien who is involved in some kind of plot that he has to hide by imposing a timed selective amnesia on himself so that while he is in the presence of their leader he has no knowledge of the plot.
Yup. He somehow breaks the neurons so he has no knowledge of the stuff he wishes hidden. In a way how they will regenerate. They kind of do a "council" thing, where they can read each others thoughts, and this is his way of hiding stuff from the leader.
One young alien escapes or runs away from his home universe and collides with a human space ship either in an attack or accident.
Kind of runs away, as he is wanting adventure. And crashes with the human ship.
The alien and one human survive but both are too damaged to survive long. The alien convinces the human to merge bodies so that they will both survive.
Kind of. No convincing required. The human is in no state to object. The alien merges with him to survive.
The human returns to earth and he appears normal but slightly bigger(and healthier?) because of the alien's added mass/influence.
Taller (extra mass) and stronger. Also no longer needs glasses
They share some kind of parasitic/symbiotic/voyeuristic mental relationship but I can't remember the details.
They talk to each other in the shared mind.
So pretty certain it is the book you were also looking for :)