1. You can add other accounts with BPS. I have 3 extra on mine. Biggest head ache is making sure that calendar appointments go onto the "right" calendar -- which for me is always the one associated with the BPS account.
2. Check the FAQ section for a document on what administrators can and cannot see.
3. I believe that the BB is more secure than an iPhone, but then I drink the Kool-Aide!
Another option for you is a hosted exchange account. That will also give you OTA access to everything. What is your current corporate email structure?