To completely flush your old coolant out you can refill the system with plain water, run the engine for a bit and then drain it. Do that once or twice and all the old coolant will be out of there.
If you're worried about the residual water in the block diluting the new stuff.. just mix up your antifreeze 55:45 antifreeze to water instead of 50:50.
It won't make a huge difference if you have a bit of plain water somewhere in the block.. but if you have an OCD fetish, just crank the engine over a few turns with the bottom hose off - the water pump will regurgitate what's left. To drain the heater is a different story involving removing bits of lower dashboard... I'd really not worry about that bit.. just use 55%..
You can re-use old anti-freeze but it's got to be spotless to be worthwhile and you have to remove all the bits of dirt and grit that you dropped into the basin you drained it into...
Use a bit of stockinette in your funnel to act as a filter. Stick the funnel into an empty plastic water bottle and then decant the antifreeze through the funnel (i.e don't pour in the whole lot.. let the "bits" settle in the bottom of the basin and leave them there with a bit of the coolant... the stockinette is just to catch any bits that fall in as you decant, not to filter the antifreeze.. ).
If you have any bits in the bottle, get a new bit of stockinette and then repeat into another plastic bottle. You'll gradually end up with antifreeze with no pieces in it. Stick that into the car.