Why does Solus just happen to look like the Ascian who is posessing him?

[SannaR]

Zenos and Ardbert were already dead. Well ok so Zenos was sort of dead. Asahi was also dead by the time Hermes was slapped into the body. I think part of why they can't take over our comrades bodies while they're alive is due to the density of our souls. We get told the reason why Lahabreha was able to possess Thancred's was due to how ragged he had been running his body. Also seemed to need the used once and then forgotten dark crystal. I think the attack Elidibus uses but fails with on Minfillia and G'Raha is how one would try said possession, but both times he's all damn your soul is too chonky.

We just don't have enough information on about Emet's possession of Solus. Keep in mind it was during a mission so for all we know he could have been on the brink of death at the time of possession. I don't think Emet would do the while still alive style as he also seems to be against the forceful memory bit when uplifting a person to a seat. Course I'm basing that on how he sounded and looked when talking about that possibility and the fact he had the Azem crystal all of ShB yet never once took the opportunity to try and force Azem's memories onto the WoL.

[Cleretic]

We just don't have enough information on about Emet's possession of Solus. Keep in mind it was during a mission so for all we know he could have been on the brink of death at the time of possession. I don't think Emet would do the while still alive style as he also seems to be against the forceful memory bit when uplifting a person to a seat. Course I'm basing that on how he sounded and looked when talking about that possibility and the fact he had the Azem crystal all of ShB yet never once took the opportunity to try and force Azem's memories onto the WoL.

Because that crystal's not one that contains memory, it contains a spell. Mitron's clearly not using the normal method, it was just the best he could manage without a crystal (which he didn't have because we gave it to Elidibus). Mitron's also an excessively terrible person in extremely weird circumstances, especially relating to Gaia, so it's possible that the method he used wasn't on the table for everyone else for both logistical and moral reasons. Personally I think logistics would be the major factor for Emet, he doesn't seem the type to be stopped by morals.

I believe it was mentioned at one stage that only the Unsundered could possess living bodies, and the lesser ones had to make do with corpses; that might've been relating to black maskers, though, and I'm not sure where to look for confirmation. It is clear that only the Unsundered have possessed confirmably living people (Lahabrea with Thancred, Emet with Solus... uhm, multiple times), so the facts line up for that, it's just that they might be coincidental.