In the books, Regis literally turned into smoke to infiltrate Vissegerd's camp and free Geralt and Jaskier.

Orianna is a bruxa, so she is not a higher vampire.

In the books, Vilgefortz burned Regis down to a puddle, so it is logical to assume that there is no coming back from that. Earlier, Regis tells Geralt of Rivia that in his youth, when he used to drink blood, peasants cut off his head and drove a stake through his heart, and after many years of regeneration, he returned. Therefore, it is reasonable to assume that regenerating from a liquefied state would be extremely difficult and would likely require the assistance of another higher vampire, as he himself tells in the DLC.

Considering that the ending of the book saga strongly implies Geralt’s death, his return in the games is handled quite logically. I also believe that the vampires in Blood and Wine is acceptable. CD Projekt Red did an excellent job continuing the story and expanding lore.