Dear
First of all, I wonder Aj Kon Klai Ban is gone!?! But later's bettern never's!
1. Aj Tnin made a very excelent discussion regarding this case in every point you've discussed. Bilateral shoulder pain in this case was releived by a unique posture of arms; it's called "Lasegue sign" with crossing arms (flexion (of elbow)-adduction (of shoulder) posture). Hence, it prefers root/plexus, compared to horn cells.
2. The disporportion between motor power of flexor digitorum of forth and fifth fingers(innervation by ulnar nerve), and of flexor digitorum of first and second fingers (innervation by median nerve). This favors plexus as well.
3. Due to hyporeflexia of right tricep, right knee, and both ankle jerks; this also a favors plexopahty/root, compared to horn cells.
4. Why only motor is involved in plexopathy? Like Aj Prasit mentioned, this is due to a special etiology of plexopathy like GBS: autoantibody against antigen of motor part of brachial plexus. Hence, in this case the etiology must not be caused by compression or trauma or entrapment or etc.
5. There are some reported cases of acute poliomyelitis caused by Poliovirus, and Flaviviruses (like Dengue, West Nile virus etc) involving only facial and respiratory muscles.
Chusana