I thought that the Crimson Stone only made him into a vampire (this is literally the only description given to it), but no such connection had been established yet.
I thought the Crimson Stone was a way to make a pact with Chaos... but that's just my Headcanon.
Then, in CVIII he goes batshit insane and actually connects with Chaos (explaining how he's suddenly gaining power to wage his war, feeding from the chaos he's causing himself).
Oh well.
Sometime after LoI but before Legends (in my headcanon they're all canon

), when Lisa, Drac's 2nd wife dies (the first being Elisabetha... it's possible that he sees something in Lisa that reminds him of her not unlike how in the Dracula novel/movie, he sees something in Mina Harker that reminds him of his first wife), that's when he loses it. At first he was just defying God because his first wife took ill. Now he's gone mad with rage seeing as his 2nd was taken from him from filthy ignorant Humans. Alucard's rage is severely supressed, by contrast, since it seems he was there when she was about to be executed, but did not go and save her since she told him not to hate humans.
I imagine he could've just shredded through them with his Half-Vamp abilities, but didn't because Lisa taught him not to act with the Rage his father had.
Right now I agree that Simon, Richter and Julius fought a fully powered Dracula (even though I can also find nothing on the matter relating to Julius, but the scale of the event makes it look like it). But Dracula's line on Judgment is really the only answer there is about his power in CVIII, and here I'll agree with Nagumo. Judgment does contain reliable information pertaining the canon (it's where the whole blood-ties connection of the Lecardes with the Belmonts came from).
If you notice, the way Drac is resurrected by people in the 100 year cycle seems to involve a blood sacrifice. This sacrifice is seen in-game in three games:
-Castlevania 1's "Castlevania Chronicles"/X68000 remake
-Rondo of Blood
-Castlevania: Legacy of Darkness.
In all of these three games, a person is sacrificed upon a font, the blood ritual brings Dracula to life.
It is possible that the so-called "100-year cycle" is just people-created... people get fed up with things and, by pure happenstance, 100 years is when they just say "Tired of this shit, let's wipe the world Clean with the searing flames of Chaos" or something.
As for Trevor needing three more allies to do it, I thought up the following possibilities:
-Trevor is the weakest Belmont to fight Dracula if we consider that each Belmont is stronger than the last. This would explain how he wasn't able to do it alone and how the others after him were.
Can we stop with this already? This was just some stupid line in the SotN Instruction booklet. It's silly and makes no real sense from any sort of way.
-Maybe he WAS able to do it alone, but the allies decided to help anyway to ensure victory, as they also had their beef with the Count.
Now this I agree with. It's possible that it's an "Avengers" kind of situation... maybe he's strong, but there are other things other than strength that would help.
-Maybe Dracula was not fully powered BUT was strong as hell anyway, and rising to fully powered status, requiring four people to put him down. Almost fully powered, perhaps?
Perhaps? I don't see why this wouldn't work.