On this site, I came across this JLPT N2 question:
これはシンプルで _______ 純粋で魅力的なスピーチです
I chose 4 while 3 is correct.
I was having difficulties choosing between 3 and 4. 1 and 2 are blatantly wrong. I eventually went with 4 because I always thought that も adds some "strength" or "emphasis", as in よりも sounds stronger than より
地球よりも大きい Even bigger than the Earth
So I thought ながらも works similarly in that it expresses a stronger 逆接 sentence.
This is an even though simple, but pure and charming speech.
I know the English sounds weird but the point is, I thought that も adds that word "even" there. I thought this would be appropriate here because the two things here are very contradictory.
I looked the ながら grammar up and all I could find were sites like this, which don't point out what different the も makes. The linked site seems to put も in parentheses as if to say that it's optional.