Truth doesn't really play a part when it comes to the DS trailer - or any film's trailer - or even in marketing in general. In product advertising the marketers pick a campaign that stresses what it is about the product that they think will get the most people to buy it. If they don't stress other aspects of the product that it might have, it's not lying. The same goes for the DS trailer. For whatever reason, Warner Brothers thinks the humor in the film will get the most people in the theaters. And because they're stressing that humor over whatever else the film may be, that's also not lying.
Also, it's definitely true that Grahame-Smith is a new name when it comes to screenwriting. As such it's not likely he made a huge amount of money working on the DS film's script, or that he's likely to make any more money off it even if the film is a big hit. And to me he comes across in interviews as someone who's more than willing to promote the film because he believes in it, not just because he has a writing credit for it - though, of course, I could be wrong and maybe all he's really doing is protecting his ass. However, whatever the case may be, this whole discussion will soon be moot because soon the film will be out for all the world to see and we'll all know what the truth is.
36 days 5 hours 54 minutes 37 seconds until the day the Depp/Burton Dark Shadows is released(ET)!!