Technical note, concluded
Okay, I think I've fixed the issue with the comments. Let me know if there appear to be any problems.
For those interested in the details: At some point when I wasn't looking, probably when Blogger changed their engine, they moved from 8-digit post ID numbers, which are reasonable, to 18-digit post ID numbers, which strike me, personally, as just a little bit of overkill, but what do I know? Anyway, in the past, the post ID numbers were passed to the comment script as numbers, and everything was fine. But now, passing an 18-digit number as a number gets it truncated. So I changed things to pass their numbers as strings, and it should work happily.