I totally agree on how ma ai is fluid and dynamic. And when you talk about a real fight on how to maintain ma ai, although we practice the far ma ai that's not usually the case if the attacker is determined to get in and corners us.
Shirata once said that in a real fight, it was vital to take the initiative to move in rather than remain static like in training. To remain passive is to be defeated when dealing with experts.
The thing is, with motion comes openings and with it comes momentum too. Morihei's post war lessons were more intuitive based, move in before the attack was even fully launched(which must then mean closing in the distance between them quickly). Also, when a defender switches from a passive stance to a suddenly fluid charging one, the attacker would be disoriented and caught by surprise.
Ma ai is just a concept to describe distance, there is no correct ma ai. Do what feels right and what suits the situation and there u have it that's ma ai. and how to obtain this feeling? Just simply as always...practice.
A very good way to improve your sense of maai and movement too is to get someone to attack you with a jo or bokken(but especially the jo). You'll get a feeling of when to move in when to move to the side and when just to retreat.
Hope this helps.