I would be happy with a clear statement from the authors with maybe an example to show intent. That is what this forum is about, imho.The fact that a number of people have a different interpretation to what you intended, is sufficient to justify an FAQ or a digital update to ensure there is no confusion and deal to those nasty rules lawyers.
From my reading of the sentence: "A battle group in the restricted area of an enemy even partly behind its rear can move straight forwards." I read this sentence to imply that enemy must have some part of at least 1 stand somewhere to the rear of the BG. Implications aren't always easy things to grasp. I think that RBS is confirming in this thread that indeed part of an enemy BG must be in the rear for this sentence to apply.
Hopefully, "REAR" is defined in the rules. (Still haven't decided to buy the format)