Hiya
A few thoughts below and again I am a fan of wrist locks, they are a part of my "game" in resistive sparring, but generally speaking I think they require quite specific set ups (usually involving strikes) to work in standing situations
Are you talking about stepping back with his right foot or stepping forward with his left foot? (I guess uke's goal is to track tori by facing him with his hips). In theory,
Stepping back with his right foot
Stepping forward with the left can work in that it allows your left hand to engage and stop tori from twisting the wrist by grabbing, but in this situation the more obvious response would be for uke to simply step to face tori instead of allowing their spine to twist
1) Uke's structure and balance should already be affected during the first step;
Firstly this is hard to do without strikes or structural redirections, unless uke is an idiot and allows their shoulders to overreach their foot
Timing / aiki will have an impact, but not much
And people recover their balance quickly, certainly faster than it takes tori to make a step, all they have to do is align their spine which is faster than uke performing a 90 degree rotation &/or taking a fairly big step
2) The atemi (in this case a punch to the face with Saito's right hand) should create space between uke's hand and his head, disturb his structure and hide tori's movement.
The strike with the right hand isn't set up at all, there is no structure to it
At best it's a feint to get a reaction, but actually uke did the obvious thing and simply covered with his left hand
The other problem with the strike is that the particular strike used encourages uke to move in the direction of the obvious counter to the lock. When you're setting up things like wrist locks with strikes you need to study how the strike compliments the lock, it's not enough for a strike to be simply a disruptive event
In practice, I don't know.
Again I'd need to try it live to answer.
Worth reiterating that, in my experience, kote gaeshi works well in high level resistive grappling sparring, but not done this way
Best