I usually just pull up the seedlings that I don't want, they grow into the lawn. I think they also spread horizontally underground. I'm not sure if roundup would travel through the roots.

I've never tried eating the seeds, but I think you have to let them get all shrivelled (the pods) and then take out the seeds.

likely they are from underground runners. roundup can affect the plants that are attached to the ones you spray.
just dig or pull them up, going back to where you want to break the run or sever the runner from the mother plant by using a shovel, then spray those.
i did not want to grow maypops for this reason but planted one in a location where it can run, just for the butterflies.
harvest them when they begin to yellow and shrivel. the juice will not be sweet and have a very "green" taste if you pick them too early.
eat them as you would a pomegranate; suck the juice and spit the seeds!