Coolant flush?

A couple weeks ago the car was running a little hot after a road trip. I saw I was a little low on coolant, so I put about 3/4 gal of tap water in the radiator (system capacity is about 3 gal) and took it immediately to a shop which put the system under pressure and found there was no pressure loss, i.E. No leak they could find. Yesterday, I tried to do a system flush to try and get the water out, but it only drained a gallon of coolant. The coolant looked like it was in really good condition (nothing floating, not murky). I ended up just replacing it with another gallon of the recommended Dex-cool coolant.

1) I'm assuming the rest of the coolant simply won't drain out with gravity. Is that the case?
2) Is there a way to fully drain it at home without a vacuum pump?
3) Do you think I need to do a complete drain, i.E. Is that enough water in the system to cause an issue?