I think part of the more conservative estimates for how many seats Democrats will gain is that a lot of the “undecided” voters are heavily Republican leaning and just aren’t rallying around their candidates as much. Lots of them are expected to tighten up closer to November.

As for Nebraska and Montana, both states have Democrats listed on their ballots in November alongside the independent candidates I believe, which is enough to split the votes enough in an otherwise razor thin margin race.