Yes, one pilot can fly it if necessary. About the only time this happens in the military is during a maintenance tests flight. Even then, usually a crew chief is sitting in the left seat.
During an emergency, it can get a bit dicey depending on what the emergency is. There is a condition where the automatic control of the engine's throttles fails and then the pilot must adjust the throttles while still flying the aircraft. The problem is, the throttles are mounted on the roof between the two pilots. Usually, the copilot would manipulate the throttles while the pilot flew. However, this is a very rare condition, so it's not like it's an impediment during normal operations. There may be situations where a Blackhawk is flown operationally with one pilot, but it won't be if it is a military Blackhawk unless somebody is doing something they're not supposed to.
Time to get airborne for a medivac bird can be as low as two minutes. The aircraft must be prepped, or "cocked", for this to happen. The crew goes out at the start of the shift, does a preflight, starts the aircraft, does all the pre-flight checks and then shuts down. Then the crew goes back through the preflight checklist right up to the "Engine Start" line and then stop. The aircraft is now cocked and ready to go. It is quarantined at that point so nobody messes with it. If it's set like that, you can go quickly. If not, then a very fast crew could have the engines cranking in 5 minutes (if the preflight was done, or omitted), and then flying a minute or two later.
I've been on the ground for less than 10 seconds while de-assing occurred, prior to taking back off. For assault missions, the less time on the ground the better.
For the Blackhawk, unless you have a big slope off to the side, you'll enter and exit from the sides. It's different from the old Huey's where you always approached and exited to the front if possible. The Blackhawk has a forward rake that tilts the rotor disk forward when it's on the ground. This means it's the point where the blades come the closest to the ground. Never enter or approach from the rear. Tail rotors are deadly.
There are dedicated medivac versions of the Blackhawk, but any model can be used for evac if necessary. Army medivac helicopters do not carry nurses. They carry a medic and a crew chief (mechanic). If you are carrying a civilian, then all kinds of paperwork and approvals are needed or you're in big trouble.
If you have other questions, let me know. I have a few hundred flight hours in Blackhawks.