Well, from what we know, the symbiont does contain all the memories and experiences of previous hosts (aka their consciousness)... but, up until now, most Trill were tasked with the premise of creating new memories for their symbionts yes... I doubt this would include the 'loss' of Grays consciousness, and we don't know if this would be considered a cultural taboo for the Trill in the 32nd century as well (considering the losses they suffered). We know from DS9 that separating the consciousness from the symbiont is possible and at the time, when Curzon/Odo expressed a desire to stay merged, the guardian mentioned that Jadzia would adapt to the 'loss', but otherwise, this merging wasn't considered taboo back then. So, at least on those grounds, I could see Gray appearing as a hologram. Starfleet could likely extract his consciousness via a neural interface and put him into a mobile emitter (which would either be the same or more advanced than what the Doctor from Voy used obviously).
As we saw, many Trill found it a taboo for a symbiont to be hosted in a non-Trill's body, and wanted it forcibly removed from Adira. The fact that Adira's situation is unconventional -- and that something doesn't have to be conventional to be okay -- is the entire point. They wanted to portray Adira as a character who defied conventional roles and prejudices, but since the future would presumably have no prejudice against their non-binary gender, the writers reflected it allegorically by putting them in a very unconventional Trill host/symbiont relationship. But post-Burn, joined Trill are much more rare, so that rarity has created a stronger taboo. The Doctor's was from the 29th century, so presumably what they have in the 32nd is considerably better.
I don’t see a need to explain it at all. How could a trill symbiant that lives inside a host and shares memories and personality traits NOT cause some weird psychological issues, especially when joined with a human. But Gray’s appearances might not be psychological at all. Could be physiological. As long as Adira is relatively comfortable with Gray’s appearances, why should others worry about it? I don’t see Stammets as “humoring” Adria so much as simply accepting her reality. Now, if we start to see Adira freaking out over it, then something will need to be done, but as of now, it seems perfectly plausible that Paul would leave her be.
Others should worry about it because it might be a sign of biological or psychological problems caused by the joining.
After all Stamets has been through with the mycelial network, I don't think he finds anything beyond belief at this point.
You could say the same thing about Paul talking to his mushrooms. Her hallucinations are really not hallucinations at all, they are the spirit of the Trill she has inside of her. Adira is learning how to be a Trill and the Trill learning about her environment through her eyes. Quite a lot of activity going there to be for certain.
You're assuming. But there's been no real medical or telepathic examination yet, so we don't know what for sure they're seeing at this point. It could be what you think, but it could also be a hallucination, or even some random alien pretending to be the character's dead lover.
Adira is they/them, not she/her. And they are hallucinations in a literal sense, in that Adira is seeing and hearing things that have no external source. But that doesn't mean they don't have a real cause. As I said, the concept of hallucination should not be stigmatized. There are a number of things other than mental illness that can induce sensory hallucinations, whether images, sounds, smells, or whatever. It's not uncommon for mentally healthy people to occasionally hear voices in their head or imagine a nonexistent presence nearby. https://en.wikipedia.org/wiki/Anomalous_experiences So hallucination is basically just really vivid imagination, or a waking dream. It's only bad if it's a symptom of some harmful condition. In this case, though, its cause is benign.
Given his husband literally came back from the dead because of space mushrooms and there is clearly some alien biology going on that Stamets doesn't fully understand, he's probably just rolling with it. Either Gray turns out to be real or it's just Adira's way of cooping, either way he's just supporting a kid who he has basically adopted at this point and just protecting them. Honestly it's probably one of the least strange things in his life.
Stamets has no reason to doubt that it's real. It's an established fact that Trill can communicate with their former hosts' personalities in precisely this way through the zhian'tara ritual ("Field of Fire"). It's slightly anomalous that it happened spontaneously, but that's no big deal given that it's the first successful human hosting of a symbiont, and that the previous and current hosts were in love, creating an unusually strong bond between them.
It’s also a pretty reoccurring theme that Starfleet officers just trust each other when one of them say something is happening. Like that alien abduction episode of TNG, Riker mentions it and they immediately start investigating it.
Didn't they get the symbiont directly from Grey? You gotta' love that they've been through enough weird to just go with it. "I'm caught in a time loop!" "How can I help?" "I've been to the future and seen the station explode!" "What can we do?" "Every one's disappearing!" "Senior staff to the ready room." "I just saw the rabbit from Alice in Wonderland!" "I'm beaming down now."