I believe you are correct that, if she wins the nomination (or perhaps even before, if she is closing it out), she chooses Brown as her running mate. I suspect several candidates have had that conversation with him already, which is big reason why he chose to sit out the primary.
Its still far too early for me to commit to anyone, but from a campaign perspective, I think Harris is a pretty good spot for the early (and most important) part of the primary. SC then NV then a Super Tuesday with CA and a whole lot of southern states…she should be the front runner after that.
Iowa is going to clean out a lot of people, I believe. There are too many people that NEED to do strongly there. Sanders, Biden, and Klobuchar, in particular all need to have 1st or 2nd place (or very strong 3rd place) placings. Beto, Booker, Buttigeig, Harris, Warren will need to competitive, too. A weak showing there for Warren for example, followed by a loss in NH is the end of her campaign. Biden doing poorly will result in massive bleeding of his support. Booker and Biden both have a problem that other candidates are better positioned in the early races than they are. By the time states more favorable to their messages start showing up, they could be scrapping along at the bottom. Folks like Castro and Buttigeig either do well early or die because of lack of funding.