We played our first game of Basic Impetus on Wednesday night - very much enjoyed it, so thank you for making such a tremendous resource available for download.

One question that came up though.

In section 5.2 it states that "Turning is only allowed forwards and rotating on one of the front corners"

We had a situation where my opponent wanted to turn an FP unit backwards, rotating on one of the rear corners, in order to protect his line from a flank attack. However, his unit was disordered.

According to my interpretation of the rules, turning backwards is, in fact, classed as Rear Movement as defined in section 5.3 and therefore his unit could not turn backwards as it was already disordered.

Is this correct?

actually turning is allowed only fowards, so you cannot turn with a rear movement.

BTW you can make a rear movement (straight), which disorders you unless you are a Light Cavalry or Skirmisher, and then (next turn) wheel. You can wheel even if disordered or you may prefer to re-order first.

You should consider Units in BI like Battlegroups in FoG or Groups in DBM/A, so not as flexible as single bases in DBA/M game.
Anyway taking on the flank is not so easy as you have to start COMPLETELY behind the prosecution of the frontage of the target Unit. A simple wheel fowards can nullify the menace in many cases.

As a tactical advice I have noticed that many gamers are too afraid of "feint flank attacks".

If you are afraid to be outflanked the best think to do is move by the side. You will disorder your Unit (unless CL or S) but if this move is performed in time it can nullify the risk as your opponent will have to make the same to try to still outflank you.