iMac ram memory installing 16GB vs 20GB
So many questions, but I'll try to break them down:


Leaving in the current 2x2GB that it has is good, it's better than
2x0GB.
All 20GB will be recognized, the only reason you'd want to replace the current 2x2GB chips with 2x8GB is purely for the additional RAM.
I recommend you pair the sticks up into the dual channel configuration for your iMac (as can be found online), so that one 8GB module and one 4GB module is on each of the two channels, enabling the use of dual channel memory speeds.