Why was Drac the only one among his friends to pretend in Griffin's obviously fake girlfriend? Well, Drac and Griffin are the only ones of their friends to not have a true love (Drac because Martha died, Griffin has yet to meet anyone). So, unlike the others, Drac personally knows what it feels like to want to be with your true love.
Dracula insists calling Dennis "Denisovich"... however, the name, apparently of Russian origin, would best translate to "son of Den(n)is". An unfortunate case of Gratuitous Foreign Language or As Long as It Sounds Foreign? Well, it may be - but in-universe! Dracula is Romanian, and it may be possible that he doesn't actually know Russian language, hence he didn't realize that the nickname he gave is actually a patronymic. (Romanian language also has patronymics, but they don't look much like Russian ones.)
If you think about it, Vlad's method of "using a kid's favourite thing to pop out their fangs" did actually work. After all, Dennis got his fangs thanks to Bela- a vampire bat. Up to that point, Dennis has probably only ever associated bats with the love and safety he feels around his mother and grandfather. To have those feelings and experiences tainted by Bela's cruelty (to his zing, no less) would've been a huge shock to his system, and possibly the kick he needed to pop his fangs out. This could also explain why using Kakie wasn't effective- real violence by someone who looks like a family member is far more shocking and disturbing than scary threats from a beloved TV character.
Of course, the simpler explanation is that it's exactly what it looks like, and Dennis did see his favorite thing turned into an object of horror, because he'd seen his best friend and childhood sweetheart being hurt.
At first, it seems Drac lost most of his CharacterDevelopment from the first movie, between his denial that Dennis could be human and the extreme lengths he goes through to prove he's a vampire, and while some of it might still be from past prejudices, Vald telling of Drac's own childhood helps explain his actions. To wit:
His insistence that Dennis is a "late fanger" is because he himself got his fangs late, meaning his denial was more overconfidence.
His drastic methods was because if all else failed, the final option, as explained by Vlad, was to more or less traumatize Dennis, which Dracula would never wish upon his grandson.
In the Fridge Horror section of the first movie, it's noted that Johnny will have to watch his beloved wife stay the same age while he ages further and further until eventually dying. This movie makes that even worse, because now he'll be forced to watch his vampire wife and his vampire son outlive him.
It is implied (through Dennis aging year by year like a real child) that Dennis may be vampire with a human growth rate, which would probably include lifespan. However, this is still Fridge Horror, as Mavis will have to watch not only her husband age and die, but her son as well.