Quotes from RetiredF4:
"After looking at the published graphs with the probable flightpath on this thread and the ongoing discusssion I offer another version for discussion. I asked myself, what kind of maneuver would I do at that point, and came up with neither a loop nor a 1/4 Clover leaf, I would have done just a low pass along the line. But the task might have been not to cross the A27 at that low altitude, requiring a pullup followed by a descent and some small heading change. I would have done a Barrel Roll type maneuver there, but never a 1/4 Clover Leaf."
"In the assumed flightpath the Jet comes in low and fast, turns somewhat to the right and pulls up, like the initial part of a Barrel Roll to the left. At that point prior pullup the airfield might be hidden behind the tree line. For a Barrel Roll type positioning the turn to the pullup point was too late, which the pilot might have recognized during the pullup. In order to reduce the forward travel he took the maneuver higher and further off to the left and ended slow on top, way off the final course and unprepared for the following descent. The reorientation to the intended flightpath while still inverted lead to an unplanned and unsafe increase in descent rate."
Unlike you, I have no FJ or aeros experience. But, as no one else has commented on your hypothesis, I'll say in all humility that it makes sense to me.
Nobody here is claiming to know the pilot's game plan as he returned towards the airfield from the north-east. At that stage, we can probably assume that things were going okay. Equally, no one claims to know at what stage his plan started to unravel, or whether he ever intended to pull up to a vertical pitch as he rolled to the left.
We know that at some point he deviated from his planned manoeuvre, but neither when nor why this happened. It could have been right at the beginning.