1. I stand corrected haha, it just looked like a force I guess.
2. Alright, I get that. The misdirections is vital for the effect.
3. If your patter is more streamlined and you stumble less, then it will be less time from when you show the six to when it changes, which will have a higher impact. If it takes a while, then the impact is weakened, at least it was for me. In my mind I see it working well like this:
Have them select their card.
Draw attention up towards your face, talking about whatever about the card and do the undercover change. (max like 5 seconds)
Look down again saying "now let's use your six...oh wait. See? What did I tell you, it's always changing up on me. Tell you what, let's use the three instead."
Perform the transpo.
As for the fourth point I made, I'm not sure of a solution yet, but I'm sure you'll be able to come up with something. Like I said before, it's a nice effect with nice timing practice. Just some refining here and there and you will have a good effect.
Jacob