tbh you lost me.
I don't know how you can honestly propose in a scifi setting that the mechanics for feeding from energy must be through physical contact ("spider-like").
Scifi is short for science fiction (emphasis on the science part) and does not equate to completely divorced from reality (which would be fantasy, and that thinking gave us Amon and the xel'naga being literal space magic gods). My idea is that protoss do have something that can be described as a mouth, just not in their heads. I was imagining something like the
Wraith from Stargate Atlantis, but it could just as easily be closed ranged like in
Dark Knight Returns Part 2 when Superman heals himself by causing nearby grass and flowers to spontaneously wilt and die. It cannot be something that would be useful in combat, though, to fit with the Zealots using weapons and physical augmentation. (One of the short stories depicts a tortured Khalai caste Protoss, after escaping containment, killing an evil human scientist by grabbing his head and using skin contact telepathy to cook his brain. Not useful in an armored, armed combat situation.)
I don't know why you deny removal of mouths* when it's the only way your interpretation makes sense. Unless you propose to change canon and give Protoss mouths? Why would you do that? Anything that makes them less like humans (or any earth creature, for that matter) is cool in my book.
*) I would also strongly argue that it would be very much possible to remove their mouths. Even on earth we have seen creatures develop lungs from gills through unguided evolution. We humans can give pigs extra ribs, we can engineer (non-functional) human ears onto rats. Why is there any doubt a race advanced enough to craft whole planets couldn't make that specific change?
Mouths are a prerequisite for complex life, so even the weirdest
speculative biology projects will depict aliens with something that can be loosely described as a mouth. For example, Project Snaiad depicts aliens with two heads, one of which has jaws and genitals and the other being a giant tongue; Nereus depicts aliens with mouth and anus in the same orifice, while genitals and breathing are in another separate orifice. Even Protoss in the SC2 retcons are described as eating with their skin (somehow), although that is really inefficient for a large animal and should realistically cause them to starve.
I don't think the xel'naga had the level of engineering you described, or at least not at that speed. If they were long-lived then they could make incremental changes that change the structure of whole worlds, but they are still subject to natural selection pressures (like the incredible usefulness of mouths). Additionally, when the Zerg started doing their crazy gene splicing the xel'naga were surprised by how unpredictable and powerful this was (in the technical sense), suggesting that the Zerg were superior at it compared to the xel'naga.
This is similar to arguments in AI research about the potential dangers of self-learning self-modifying AI advancing beyond our understanding and control. The Zerg are a classic example of a "paperclip maximizer," although they cheat by being biological entities with fundamentally biological motives like survival and domination.
I don't know why you state that you don't like solar powered beings and in the same sentence you say you prefer plants. Plants are solar powered, specifically using photosynthesis.
Also who said Protoss aren't psychic plants? I think your definitions are too narrow and "earth bound". Allow yourself to widen the possibilities. We're in a scifi setting.
"Scifi" isn't the same as throwing all sense of logic and any basis in real science aside. I am quite familiar with speculative biology and that is why the Protoss biology (specifically the SC2 retcons, not their vague SC1 appearance) peeves me.
Normal plants are solar powered. Animals that evolved from carnivorous plants would lose their photosynthesis. If Protoss evolved from carnivorous plants (or siphonophore jellyfish, as I suggested before), then it would make sense they would have mouths somewhere other than their head.
It is highly unlikely that any organism with a head would not have a mouth on it, as every single organism on earth with a head has a mouth on it (a good example would be cephalopods like octopus, squid and cuttlefish). For the sake of argument we can assume that Aiur had the perfect selection pressure for that. Although that doesn't explain the bengalas, which is virtually identical to an Earth panther and only makes sense as an introduced species (which actually fits quite well with the Protoss being a space empire so whatever).
I can agree on the last part though. I wouldn't mind their reasoning to be more alien.
I always thought the Protoss were too human-like in appearance and psychology. Their skull structure alone doesn't make much sense (although it has changed dramatically in different art): why have a jawbone if you lack a mouth? Is it supposed to support nostrils and/or gills? Looking at the portraits on battlenet: I'm pretty sure the Arbiter has gills and the Scout has a nose with a bridge and nostrils.
Compare that to the
R'ha in the short film of the same name: their skull structure is elongated in comparison, less wasted space and seemingly makes more sense for an organism without a mouth in its head.
Also, I have not pointed out the biological implausibility of the Zerg. Their biology is pretty insane, but what surprised me is that some of it has a basis in computer and engineering science. The creep is an all purpose ecosystem that digests raw materials, is probably photosynthetic (it's even grey like photovoltaic cells), and acts as a circulatory system that provides nourishment to buildings and units alike.
None.