I'll go with Gaahl's format!
#1 Silly to neglect this simply because of time constraints. As Gaahl said already, it wouldn't add too much to the running time so if this really were the case it wouldn't be justifiable.
#2 I almost agree with this, but the way it's phrased, it sounds like I'm blindly saying "I don't care as long as I see Berserk again!" And with a story element such as this, I do care.
#3 Obviously, I wouldn't prefer to have them add this story element later on in the trilogy, but if they somehow manage to weave it in in a great way, then yes, I'd be okay with it. I won't pass judgement against it until I watch all 3 movies.
#4 It won't ruin the experience for me. There are so many reasons why I love Berserk, that I don't expect these movies or anime to ever tug on every heartstring that I have attached to this series. So yes, the story element will be sadly missed, but it's not a deal breaker.
On a side note, (I know this thread is just speculation), but I do have a gut feeling it will be in there. I really just don't think Gut's childhood is necessarily trailer material, more specifically, theatrical trailer 2's material. With the narration, and the focus of this recent trailer, they were going for the unique relationship that Guts and Griffith share, with sprinkles of emotional camaraderie. I agree that the poster doesn't show anything in regards to Gambino, but I still am not going to count it out. Anyways, I'll just keep the faith!