Is there a macro command for this...

[Callinon]

For the moments where between the time you press it, and the time you iajutsu, something happens that necessitates backing off. Before, I'd have a 5 second leeway before I run the risk of losing the 20 kenki for nothing. Now, looks like I'm separating the buttons.

Using separate buttons is the correct answer. You're losing almost a full second of uptime every time you use iaijutsu with your original setup. That's a full GCD or more every two of those.

In general you should not macro things you need to press a lot. I know that sounds counter-intuitive but with the way this macro system works it is the truth.