Is a 70B ever a Platoon Leader?

Absolutely. In my unit there's a slew of lieutenants and my commander makes each one hold the position of PL for a year or so.

Absolutely, if you branch Medical Service Corps you will be a 67A with a track of 70B. You can serve as a Treatment PL, Evacuation PL, Ambulance PL, Forward Distro PL. I was a Treatment PL with a Sustainment BDE. It was my job to manage the 68W (Combat Medics) and ensure they maintained thier EMT Certs, conducted Sickcall, and provided Force Health Protection. It was an enjoyable Leadership position.

Yes. That should be your very first assignment as a 70B.