His description of what he wants is pretty much 5.7 ballistics. If he can get there with a 223, what's the point of more expensive 5.7? And why do you think the same bullets and power out of a 223 case is "decreased lethality" compared to a 5.7x28? That cartridge isn't magic, and a 40gr @ 2,000 fps from a 5.7 does the same thing as the same 40gr @ 2,000 fps from a 223.
OP, what you want is easily achieved from the 223 round, especially with Blue Dot which is great for reduced 223 loads. However, making the rifle cycle is a different story. You keep mentioning that you don't want to rebuild the gun, but you may have failed to consider the basic physics of the operating system - a rifle that runs normal power 223 correctly is unlikely to run severely underpowered loads reliably, if at all, without tuning/modifying some things. And conversely, a rifle set up to run those light loads will probably be unreliable with full power loads, and possibly even unsafe.
Besides, even young kids can shoot a 223 well; it doesn't recoil that much. My own kids can attest to that. I'm guessing you're maybe pretty new to this, and mainly are reacting to the muzzle blast. A suppressor is the obvious answer, but even simpler is simply doubling up with ear plugs and ear muffs; that may sound too simple but try it and you'll likely find that 223 is very manageable even for smaller people. Also maybe a blast diverter would help you. I really dislike those, but in your case it may help.
