Thanks for trying to explain it Chris, well, I won't make a report of his driving but I'll try to answer why his car spins out of the track when he's braking, likely will be the brake & throttle overlapping, as you said.
Brake & throttle overlapping not just helps on balancing the weight transfer of the car, this helps too improving vehicle stability when downshifting gears and it keeps your rpm's higher when you exit the curve, this means more power at your disposal.
I'm applying rather a 15~25% of throttle than a 50% , more than that, the braking won't be effective.
I got the bad (or good) habit in iRacing, where some cars (if not all) are really difficult to handle when braking and downshifting, there, heel & toe is a must learn if you want achieve fast times, in GT5 or GT6 it's not needed as we have auto-blip, but adding a bit more of throttle helps none the less, in almost every replay you can find from me in GT5 or GT6 (since one year ago or so) you'll see I always give a bit of throttle when braking.
About if this technique is realistic or not, it's quite clear to me.