Reasons why Deloria shouldn't be able to be Silver:
Deloria attacked Gray's town and killed A LOT of people, including his parents. Gray was found by Ur and spent weeks/months/years training under her, during which time Deloria presumably would've continued attacking other towns.
While Gray was learning magic and Deloria was rampaging, Silver's corpse would've decomposed or been buried - but needless, Deloria should've long left it behind and shouldn't have had a reason to notice it at the time of the attack in the first place.
After such time passed and Deloria is confronted by Ur and her students and is sealed, it was either frozen alive in the ice the whole time until being unfrozen and 'dying' on Galuna Island, 'died' instantly upon being sealed by Iced Shell, or 'died' after having spent some time in the ice and was already dead upon being unsealed.
If it was in the ice alive the whole time until it was unfrozen by Lyon and it died there, there's no way it should've been able to get Silver's body 10 years after killing him. If it died instantly after Ur used Iced Shell, there's no way the place where they fought him was near where Silver's body would've been for him to jump into it - location and time (body left there while gray trained) wise, that's too unlikely and would be BS. If Deloria died some time after being frozen, then the other 2 points still apply.
The only way I think the author could think this twist works, is if he's implying Deloria noticed Silver's body during the attack on Gray's village and took it as a human form then - and its just a coincidence that the body would be the father of the student of the teacher that sealed/killed him.
Otherwise, Ur's battle with Deloria happened to take place very close to Gray's village and after being sealed/killed, Deloria discovered Silver's body and decided to possess it and later get revenge on Gray about 17 years later (10 years since Ur sealed him + 7 year time skip)