@narenn
Can you kindly look into the following" official" exaples on this issue?.
GPrep sample
Dressed as a man and using the name Robert Shurtleff, Deborah Sampson, the first woman to draw a soldier´s pension, joined the Continental Army in 1782 at the age of 22, was injured three times, and was discharged in 1783 because she had become too ill to serve. --- Correct Answer is A.
A. 22, was injured three times, and was discharged in 1783 because she had become
B. 22, was injured three times, while being discharged in 1783 because she had become
C. 22, and was injured three times, and discharged in 1783, being
D. 22, injured three times, and was discharged in 1783 because she was
E. 22, having been injured three times and discharged in 1783, being
You may see that the correct answer A uses a comma before the fanboy, but still does not repeat the subject Sampson;
OG example
The root systems of most flowering perennials either become too crowded, which results in loss in vigor, and spread too far outward, producing a bare center. Correct answer is D.
(A) which results in loss in vigor, and spread
(B) resulting in loss in vigor, or spreading
(C) with the result of loss of vigor, or spreading
(D) resulting in loss of vigor, or spread
(E) with a resulting loss of vigor, and spread
Here also the correct choice uses a comma before the fanboy (or) and still does not repeat the subject the root systems after or.
This is the GMAC thinking as far as I see;
I have no idea of Kaplan’s thinking on this point; but while I do regard your point, IMO, commas aren’t that great deciders in GMAT;