I am not to sure the NGs will make seed. I have never gotten any from them but I think they should. To try take the powdery stuff off the male flower and place it on the tip of the female flower. That is the part that looks like a nose or a finger.
Impatiens balsamina blooms differently where ever it is growing for you in Zone 11 I would think almost all year. Pulling out some where they are not wanted wouldnt hurt. If you do it carefully you can plant them where you do.
Most doubles you buy comerically dont set seed. There are seed packets you can by of double but its not 100% but the chances are good. I would guess after a while the seeds they may produce will be single or simi double. Its not a mutation that is very strong.

Thank you Mrimpatiens. Unfortunately one of my New Guinea impatiens is not very healthy. I am not sure if it is because of the heat. I do water regularly, so I doubt I will be getting seeds from that one.