You do know that Thermold mags are still STANAG mags?
STANAG-4179 does not determine the shape, size or capacity of the mag; it only determines the ammo a magazine holds; in this case is a 5.56mm NATO.
A Beta C, box mag and a drum mag are all STANAG mags. That's just being annal.
How do the RET (buffer tubes) attach to the lower receiver?
If they do screw into the lower, and it can take AEG tubes, then why would it not be able to take real stocks? AEG stocks are very close to milspec stocks. IF the RET screws in and the threads are the same pitch and size as a real tube, you could buy a RS stock set and put it on the GBBR.