That was a really old theory from 1.0 that has since well and truly been disproven - not only did Zenos (the Legatus of the XIIth Legion) have virtually no connection to Vayne or the Archadian Empire from FFXII, or, anything from FFXII really, the Return to Ivalice raid introduced none other than Gabranth in identical armour to his FFXII version (or at least, a direct Garlean copy of him) as the Legatus for the IVth Legion. So, I think we can well and truly put a well deserved bullet behind the ear of this theory.
EDIT: Okay I guess Zenos did take control of Shinryu (essentually becoming a dragon of sorts) along the lines of how Vayne became the Undying, but even that's a big stretch.
Re: Solus. The original body Emet inhabited as 'Solus' really was buried/intered/cremated or whatever funeral tradition the Empire has, they had just cloned him beforehand, which is the body he was using throughout ShB (this was demonstrated towards the end of the SB patch story when Emet was first introduced - Varis shoots him dead, only for him to instantly show up in a brand new body identical to the one just killed. Gaius also confirms that the Empire has been using Allagan cloning technology at the facilities where Black Rose was being produced and that the cloning capsules were all filled with cloned bodies of "a young Emperor Solus".).
So it's pretty clear that the 'original' Solus is dead and buried, he just had himself cloned, and it is these cloned bodies which Emet has been using ever since (it should also be noted that it's only recently that Emet started 'body-surfing' unlike other Ascians, he outright mentioned that when an Ascian constantly switches bodies it actually drains their power, which incidentally explains why he was so much more powerful than Lahabrea had been despite both being 'unsundered' Ascians - Lahabrea was constantly jumping from host to host, probably for countless millenia, and that it affected him to his detriment (according to Emet anyway).
Emet however seemed to prefer to live in the one host from their birth (or at least from early in their life) through to their natural death and so was able to better preserve his power, which is why as Hades he was so powerful (and how our usual tricks when dealing with them... didn't seem to work). He had only now started moving into his clone bodies after his original 'Solus' body finally died from extreme old age, but still had all that power to burn accordingly, which he did at the Dying Gasp. But I digress...