1. How big is the orb? How long would it last if they defend with an armor type jutsu like Hardening Technique, How badly does it damage them and how fast?
2. So if they turn around they'll see what was naturally behind them before or would their vision be a locked image of what they were looking at when the gen is cast? I'm guessing the 1st.
1- It'd first be as big as the opponent, since it has to form around them, but the moment it does that, it begins to close in, causing them to lose the ability to move their arms/feet to move properly, which is why they can't use hand seals upon being trapped in this jutsu. If they used Hardening technique, their armour would be able to break the Orb whilst breaking away, so, at the end they wouldn't have an armour but the orb of wind would be gone too (since Wind and Earth are neutral).
2- ''Furthermore, the user would manipulate the chakra further, so that if the opponent turns around, it'd still look exactly as it was, due to the user linking his vision within the chakra, and making his opponent see the same thing he was seeing.''.
In short; they'd see the area behind them exactly as it was before you put them in the genjutsu.