It is eternal and never changes.
Okay, lets start from this premise and see where it takes us.
All things are shaped by it.
Shaped is a discrimption of change. This can't be true if your first statement is true.
Time is an illusion. Time = change and proto-consciousness never changes. Change is also illusory since time doesn't exist.
Okay, these are in line with you first premise.
Monism is true - there is only the most fundamental,
Fine.
and all things are built from it,
Oh, no. Without change things can't be built.
like a sand castle being illusory and combined of individual grains. You can destroy the castle, but the grains never changed at all, only their apparent location and relationship to themselves changed.
Minor quibble, but grains of sand do change.
Proto-consciousness has no boundaries or inherent limits,
Okay, that is convenient. That also means it has no definition.
nor does the things that it binds up with itself to form, such as a multiverse or reality in general.
With out change we can't have bind or form.
The universe is infinite and eternal because it is derived from, and literally is an emergent form of the necessarily eternal, fundamental proto-substance.
If there is no time we can't have infinite or eternal either, both are part of time.
Nor can we have something be emergent if there is no change.
Consciousness happens when proto-consciousness is structured in the right way. Its a fundamental property of itself and is self reflective.
Things can't get structured if they can't change. All things are a fundamental property of what it is since it can be nothing else.
I cut the rest of the argument off, since at this point we can't really have anything but the fundamental. There can be no higher order, no emergent properties since nothing can change.
The fact is for something to happen, somewhere there must be movement, and if there is movement the must be time for it to occur in.