[Yoda]

It is my understanding that the right forearm is only on plane at address/impact and as it paddlewheels into impact.

Agreed . . . The Right Forearm can be On Plane only when the Right Elbow is On Plane.

. . . it [the Right Forearm] . . . ends up perpendicular to the inclined plane at the top of the swing.

The alignment you've described requires a 90 degree bend of the Right Wrist. Though this extreme is possible, it is not normal. See my last comment below.

The left arm flying wedge is on-plane at the top of the backswing . . .

No way (except with Zero #3 Accumulator and its theoretical Left Shoulder Plane Angle). The two Wedges describe respective Arm-and-Clubshaft alignments. Neither is On Plane at the Top. See Photos 9-3-6 and 10-6-B #2.

. . . but the right forearm flying wedge is at a 90 degree angle to the left arm flying wedge.

Agreed, but this alignment long has been misunderstood by TGM students. What it means is that the respective Wedges with their own unique alignments are brought together in a 90 degree relationship (as in palm-to-palm). It does not mean that the Right Forearm is perpendicular to the Left (at the Top or otherwise). Instead, it comes in at the angle dictated by the Right Wrist Bend (as established at Impact Fix), just as a supporting strut angles into an airplane's wing. Only a 90 degree bend -- possible, but extreme -- will produce a 90 degree relationship between the Forearms.