this is how i think the timeline will work out:
1. gabriel becomes a vampire post-LOS DLC
2. trevor is raised by the order and his identity is kept secret from gabriel, as is gabriel's from him
3. gabriel occupies bernhard castle and begins to gain a reputation as "the dragon," dracul
4. trevor is trained by the order with the ultimate goal of defeating dracul
5. trevor fathers a child at some point, thus continuing the belmont line
6. trevor invades the castle in MOF, fights dracul, discovers he's actually his father, and is turned
7. knowing that he's "the son of the dragon," dracula, he mirrors his name in protest and becomes alucard
8. dracul continues his reign of terror, but alucard returns in LOS2 to try and take him down again
9. the belmont line continues through trevor's offspring, eventually giving birth to simon
10. in the distant future, weary of centuries of fighting his descendents, dracul enters into a final battle with the newly-resurrected satan (LOS3?)