1) can Dracula use DB on last card on his trail ? Rules doesnt forbid this, but it seem little weird, since he doesnt "cross" his trail by staying in same city ... but i suppose it is a correct move anyway?
I think that's perfectly legal. A bit odd, but otherwise fine. It should be pretty obvious to the hunters what you've done, so it wouldn't be wise to do this if they were getting close to you. It also means your DB card will be out of commission until it drops off the trail. If you're all out of other cards like Hide or Feed, then fine, but otherwise I'd say use a different card and save DB in case you need it. That's a question of tactics, though, not whether it's legal.
2) When Dracula is forced to tell his next move, lest say the day/night marker is on second place at Day, so two moves is neccesary to night, can Dracula say he would use Wolf Form (or any other power card usable only at night) and say he move with this to city 2 spaces away ? Assuming he has Event card that lets him move day/night marker one space forward... And in a situation the hunters cancel his event card, he can move whereever he wants, since the effect that forbids him to do the "promised" move comes from hunters?
Yes, I'd say that's fine as you wrote it.
Another question (the answer to which I am unsure) is whether or not Dracula would be required to reveal his Event card to explain how his declared action will be legal?
I think the answer is no, all he would have to say is what his next move will be, and assure the hunters that he has the means to make it legal. If it was later proven that he was lying about having said means (ie: he didn't really have the event card, just pretended he did), of course, then he would be cheating and lose his turn when he failed to make the "legal" action he had declared.