As for the trim of the boat, the main thing is that you are thinking about it. The exact placement of the kids and gear is something you will figure out, and it need not be perfect. Trim is a problem for many people who just don’t think about it at all, but that won’t be an issue for you. Be as fussy as you want about trim, or just moderately fussy, and things will work out okay.
Your plan for rowing from the stern (with the boat going backward) sounds okay. You’ll probably find that whatever height off the floor you can provide will help. Most people find the outstretched leg position with no back support to be awkward, and the lower the seat, the harder that will be. For what it’s worth, in my dedicated rowboats, the center seat (which use when rowing solo) is roughly 4 or 5 inches above the bottom of the hull, and that works fine for me.
I also think mounting oarlocks on the gunwales is the way to go. You can always change things later if you think an outrigger setup would be better or that becomes necessary due to raising your seat height.
That first link you provided looks like a good method to use for a fixed oar position in the oarlocks. Another traditional method uses a pin that goes right through the shaft of a wooden oar. Many oars come pre-made that way. The second link uses a method that is more elaborate and designed for use with oarlocks that would otherwise allow feathering on the recovery stroke. A purist would say you should feather, but I don’t think most people need to be that elaborate (I use pinned oars, that is, no feathering).
As far as overlapping the handles, if I understood you correctly, no, you don’t want to reverse your hands (right hand to left oar, etc.). Doing so would eliminate crossover at the center of your stroke but create extreme crossover when pulling back and reaching forward. With a little practice, crossing your hands ends up being as automatic as walking. I typically alternate which way I cross over on each suddessive stroke, though often, if the wind creates the need to row more strongly on one side, I cross my hands in the same manner for every stroke. Check out these videos. For the upwind clips, one hand or the other, or both, might go higher than normal to make sure the blade is deep enough to be below wave troughs. In calm-water rowing, my hands tend to be lower which creates more crossover, and the hands just brush past each other on the crossover. You’ll see that in some of these clips:
https://www.flickr.com/photos/11908573@N00/albums/72157626731449520/with/5728460843/
Just for perspective, that boat is 15 feet long, 38 inches wide, and has the same 12-inch depth at center as your Grumman, even though the scale here is rather distorted by the wide-angle lens.