I just did the same again, thinking something may have unblocked it. It says 15 available, but when I click it only 5 show up. Of the 35 comments, only 15 show up,
" ... they do not expose the conversations implied by the number to the right of the word DISCUSSIONS and COMMENTS".
I don't think your interpretation is correct.
"It says 15 available, but when I click it only 5 show up".
Which should be explained by your subsequent comment:
"In the 5 threads under My Discussions, I have posted exactly 15 times, including the original posts"
You had 15 posts in the 5 discussion that you created; seems to me what should be expected.
"The 'My Discussions' button seems to expose only those discussions that I initiated, but NOT other discussions where I have commented, right?"
Correct - as you already noted.
"Is it possible to know what other discussions I have contributed in?"
Yes. Click the "Comments" button, just below the "Discussions" button.
"Discussions" refers to the discussions you initiated.
"Comments" refers to the discussion you did not initiate, but to which you contributed one, or more, posts.