It seems so you can go as wide as you want in the elbow distance provided that you make the distance between your feet as wide as possible. So, what is the ideal width between elbows? I find keeping it shoulder width makes my stance go a bit wide... and I am not sure if that is good..?
"Ideal" would be the width that allows your upper back to keep in extension, shoulders externally rotated, scapulas back. Too narrow would cause your upper back to go into flexion, robbing you of one of the key benefits of the zercher squat - upper back stability.
Wider stance is perfectly fine for squats, joint health-wise, assuming you keep your knees out and don't let them fall inwards towards each other.