A lot of people will tell you that discs work better than v brakes in the rain, I have'nt had a problem with my brakes rain or shine. If there are not very many people on the sidewalk, and assuming it is wide, I wouldn't be shy about using it if I'm going that slow. Where I'm at there are areas where I hardly ever see anyone on the sidewalk though.
A more aggressive riding position may help, it's important that you're comfortable too. Doesn't the 7100 have an adjustable stem? My 7300 had an adjustable stem and I played with different positions alot when I first got it. My Novara Buzz V happens to have the same position I settled on my 7300. If you find MTBs comfortable then maybe your search is over. On my 7300s sus fork I tightened it down so it felt rigid but it gave on larger bumps and such, that way it wouldn't bob around when I stood up on the pedals. A lot of people use hardtail MTBs and put 26 X 1.5 tires on them for commuting. There are a lot of used MTBs on Craigs List, and a good chance your LBS has one available.