DG - Your answer's reasoning/logic does not resonate with me.
I will provide my personal answer to those two questions.
I believe that the main reason why the trail forearm moves increasingly towards a greater degree of supination between P4 => P5.5 in most pro golfers is due to the clubshaft shallowing effect.
Here are capture images of Henrik Stenson's P4 => P5.5 downswing time period.
Image 1 is his P4 position. I suspect that his trail forearm is minimally supinated at P4 because his clubshaft is pointing left of the target. If his clubshaft was parallel to the ball-target line at P4, then his trail forearm would likely be more neutral - presuming that his trail arm is roughly at that same P4 position. Note that his trail palm is parallel to his lead palm because he uses a neutral lead hand and trail hand grip.
Image 2 is at his P5.5 position, and one can clearly see that his clubshaft has progressively shallowed to a shallower swingplane between P4 => P5.5. I think that his trail forearm has to be markedly supinated at P5.5 to allow his trail palm to be parallel to the swingplane on which his clubshaft is positioned at P5.5, and to also remain parallel to his lead palm.
Then, watch what happens between P5.5 (image 2) and P6.5 (image 4). Note that HS is releasing the club (releasing PA#2) due to a lead wrist uncocking action that is happening in the plane of his intact LFFW. Note that there is a small amount of counterclockwise rotation of his lead hand happening due to the slow phase of lead forearm supination that allows the clubshaft to continuously remain parallel to the functional swingplane between P5.5 => P6.5. During this P5.5 => P6.5 time period when the clubshaft is undergoing a conical pendular motion, note that the arc of the clubshaft motion that keeps the clubshaft on the functional swingplane (=on-plane) goes under his hands in an in-to-out directional manner. The trail palm has to remain parallel to his lead palm (and also the clubface) during the P5.5 => P6.5 time period, where it is under the clubshaft at P5.5 and partially under/partially behind the clubshaft at P6.5. That is only possible if his trail forearm remains supinated - considering the fact that there is no significant change in his degree of trail wrist extension. If his trail forearm became frankly pronated between P5.5 => P6.5 it would induce a "tumbling action" that would throw the clubshaft over-the-plane and induce an out-to-in clubhead path.
Here are capture images of his P5.5 => P7 time period as viewed from a face-on viewing perspective.
Image 1 is at P5.5, image 2 is at P6, image 3 is at P6.5, and image 4 is at P7.
I have drawn a blue line down the middle of his trail antecubital fossa and a red line over his trail forearm's radial bone - which shows that his trail forearm is still supinated between P6.5 => P7.
Note that the back of his lead hand is rotating counterclockwise between P5.5 => P6.5 due to the slow phase of lead forearm supination, and that most of the lead forearm supination needed to complete the release of PA#3 happens between P6.5 => P7. During this P6.5 => P7 time period, the club handle/clubshaft is rotating at the same rotational speed as the back of his lead hand because he uses the intact LFFW/GFLW swing technique. During this P6.5 => P7 time period, the trail palm remains continuously parallel to his lead palm and the back of his intact LFFW. Three biomechanical factors interact to make this possible - i) the trail forearm angle (which is dependent on the degree of internal rotation of his trail humerus, the position of his trail elbow and the degree of trail elbow straightening); ii) the degree of straightening of his trail wrist; and iii) the presence of any trail wrist circumductory roll motion. Note that his trail forearm has to remain supinated during the P6.5 => P7 time period because if his trail forearm became frankly pronated it would induce his intact LFFW to roll excessively counterclockwise and produce an excessive clubface-closing action.
I have never seen a pro golfer who pronates their trail forearm between P5.5 => P7, and they all have supinated trail forearms. The degree of trail forearm supination existing between P5.5 => P7 will vary and two major trail upper limb factors will affect it - i) the position of the trail elbow at P5.5/P6 and ii) the trail hand grip strength.
Consider two examples.
Dustin Johnson
Image 1 is at P5.5. Note that his trail elbow is positioned in front of trail hip joint area and that his trail humerus is externally rotated. DJ also uses a strong trail hand grip that will position his trail palm in a more clockwise rotated position relative to his lead palm/club handle. That combination will require more trail forearm supination to exist at P5.5.
Note that his trail forearm is still markedly supinated at impact - and that is because he uses a strong trail hand grip which positions his trail palm more under the club handle. Also, note that his trail humerus moves from being externally rotated at P5.5 to becoming neutral at P7, but it never becomes internally rotated. That means that his trail upper forearm is not rotated as much counterclockwise and that makes it easier for him to keep his trail forearm supinated and his trail palm more under the club handle at impact.
Note that DJ uses a moderately strong lead hand grip, so he does not need to rotate the back of his lead hand as much counterclockwise between P6.5 (image 2) => P7 (image 3), so he consequently does not need to rotate his trail palm by very much during the P6.5 => P7 time period. If he frankly pronated his trail forearm between P6.5 => P7 it would result in an excessive clubface-closing action.
Now, consider Rory McIlroy's trail forearm motion between P5.5 => P7.
Here are capture images of Rory at P5.5.
Image 3 is at P5.5. Note that I have placed a blue circular marker over his trail palm area and a red circular marker over his trail antecubital area that shows that his trail forearm is not angled away from the target (compared to Dustin Johnson). Note that his trail elbow is situated away, and slightly behind his trail hip joint area, due to his use of a punch elbow motion between P4 => P5.5. Note that his trail humerus is internally rotated to a small degree at P5.5. Rory also uses a weak trail hand grip. Those biomechnaical factors would cause his trail forearm to be less supinated at P5.5 (compared to DJ) - even though they have a similar angle of the clubshaft relative to the ground at P5.5.
Capture image of Rory McIlroy at P6.
Note that his trail humerus is internally rotated and that his trail hand is very slightly closer to the target than his trail elbow.
I have drawn a red line down the middle of his trail antecubital fossa and a short blue line over his trail lower forearm's radial bone so that one can roughly assess his degree of trail forearm supination.
Capture image of Rory McIlroy at P6.5.
Note that his trail humerus is more internally rotated which angles his trail forearm towards the target.
It looks like his trail forearm may be slightly less supinated compared to P6.
Capture image of Rory McIlroy at P7.
Note that his trail forearm is still supinated, but less than at P6.
Two factors determine how much he needs to straighten his trail wrist in order to allow for his small amount of needed clubface-closing action, which is less than usual (eg. Henrik Stenson) because he uses a moderately strong lead hand grip - i) his degree of internal rotation of his trail humerus and his degree of trail elbow straightening and ii) the fact that he uses a weak trail hand grip, which means that his trail palm must be positioned more behind the club handle at impact (compared to Dustin Johnson who has his trail palm positioned more under the club handle at impact). So, it is not surprising to note that RM's trail forearm is less supinated at impact - compared to DJ's trail forearm. However, his trail forearm cannot become frankly pronated between P6.5 => P7 because it would result in an excessive clubface-closing action.
Jeff.