I say if you had to, yeah you could solo heal with Moira. Depends a lot on the team comp, cause in dive: absolutely not. But if your team is close enough together, she'd be extremely effective!
It doesn't matter. As long as the healer dies. You will have no source of healing except of the health packs. Second healer is always recommend in any situation.
Moira has burst healing similar to genjis burst damage. Ride the back line going into a team fight with a second support. Spray your team as they engage and let a death orb go into the crowd. This method works very well. You need the second healer however.