Ok, a few notes on this, someone let me know if Im wrong. First of all, when doing a down b, you have to pull back first then do the down b right after, not thunder and then back. Can someone please confirm this for me?
Second, this sort of applies to your post in the game techniques twinkle toes for standard b. At least for pikachu, if you press backwards around the time the thundershock is coming out, then you'll move backwards like you said. However, if you press backwards just slightly after you press b (just late enough to avoid doing a side b), then your thundershock will also turn around in the other direction just as if you were on the ground in addition to moving in that direction. I can't remember exactly, but I think that last part about your thundershock turning around in the air was also in melee.
One final thing, and this is an incredibly weird glitch. I tried using the c-stick for special attacks for QAC, and was trying this out in training. When you use the c-stick (and this only works for the c-stick) to thunder and do backwards movement, you start out with your backward momentum, but the second you hit the c-stick all of a sudden your momentum changes to what it originally was (forward). Depending on what point you allow your momentum to change back, you'll either end up on the side of the thunder you would have with your original momentum, being hit by the thunder, or if you do it really late still on the side of your secondary (backwards) momentum, but closer to the thunder. But here's the thing. This only works if you hold onto the analog stick the whole time. If you wanted to, you can also change your momentum (backwards), let go of the analog stick, and doing thunder with the c-stick will still leave you with your secondary momentum. It's hard to describe unless you try this all out yourself.