Wow. Having started out a year ago with a Scout engine on a layout on a concrete basement floor, with 027 curves, I fully understand your "fly off the track" issue.
Yet, in my opinion, and based on my experience, you are utilizing very complicated electrical solutions for something that is substantially curable by simple mechanical adjustments.
1. 027 track is too tight to use as your curves and is not as rigid as the 031 type track. Use 031 type track and go up to 042 curves. You could go up to just 031 curves and put a straight piece of track in the center of the curve, separating the two halves of the curves, to reduce the centrifugal force that causes the roll overs.
2. Having a layout "on the carpet" is fun, but the track will never have a solid foundation and will always wobble and flex when the engine rounds those curves. As a matter of fact, it will bend outward when the engines comes around the bend and cause it to roll over. Screw it down on a homesite or plywood board, 3/4 inch.
3. Absolutely use the 1033 transformer. You certainly don't need any more than that. It is the F-150 of train transformers. And yes, connect the lower voltage posts to the track.
4. Find the highest throttle position on the transformer, where the engine will always stay on the track. Buy a peel and stick blaze orange sticker (such as the two inch round target stickers sold at gun stores), and cut it into a large red arrow or marker, and stick it to the face of the transformer to mark the "too high speed" threshold. Any child old enough to work a throttle by hand is old enough to learn that going past the red marker will roll the engine. (In my opinion, the entire purpose of a train set for a child is to teach them how to operate mechanical devices.) Believe me, your grandson will figure this out in about 5 minutes, particularly if you make him put the engine back on the track himself when he rolls it.
5. 90% of the problems you are having is because you (like me) are starting with a high speed low-cost Scout engine (I only paid $60 for mine), and you are attempting to design and wire your entire layout to cure the speed issues that are endemic to those inexpensive models. When you feel comfortable about it, move your grandson up to a used 1970s type loco, with a well operating Pullmor motor. These run much more smoothly, and diesels with these motors can be had for around $75.
I bought and used my Scout primarily as a cheap energy-hog "test engine" in building, wiring and testing my layout. It worked great for that. When I was done, I sold it for $10 less than I paid for it. I know you put alot of work into yours, so keep it on a shelf for display and occasional use. It will last forever and maybe your grandson will keep it later in life as a keep-sake.
If you think back about it, for all of the extra money and components etc. that you have spent on rehabbing the Scout, and adding electrical components, I am pretty sure that you could have bought a used VG+ Thomas the Tank Car loco, with a modern smooth running can motor, and your grandson would have probably thought that it was really cool.
Hope this advice helps. (I am still learning as I go.)
Mannyrock
