Yeah I think it is good to check a few summaries from there to get a feel what it will be like. Im not sure what dark comedies you have seen in the past but in my experience, there often is a sadder element to them too

I think it depends how much information you are looking for going into the movie. Rotten tomatoes tends to be what I use.

Trailers are not a good way to gauge what a movie will be like as they are designed to appeal to as broad an audience as possible. Hopefully you have learned a lesson for the future