“I checked the lyrics in the actual album and they say “cockle shell.” I know that The Wall lyrics sheets are notoriously unreliable, and even have songs that aren’t on the album. Did they get it wrong, and then since Dave wasn’t the original singer he relied on that when he went to learn the lyrics?”

I’m less confident now though, after checking every commercially available performance with Waters. Of the four commercial releases where Waters sings it, I can’ maybe here a second syllable in them movie, but not the other three. But I assume that if he ever said “cockleshell” the he always said it.
There are two more where his backup singer takes the line, the backup singer on the 2010’s Wall tour definitely says “chockleshell.” I noticed it on the official release, but I checked some good quality bootlegs on youtube and you can clearly hear it every time. I hadn’t checked that one before. I really can’t hear it on the album, the live album of the original tour, Berlin ‘90, or the recent tour with the other singer handling it.

Complicating it is that “conch” has two distinct pronunciations, one more widespread (contch) and one more correct (conk, it being a Greek word).

I didn’t know this, tbh. I’m sure that I originally learned the word from Lord of the Flies so I presumably mispronounced it until someone corrected me, but I have no distinct memory of thinking that the one that doesn’t sound like “conk” was “more widespread” or anything but a mispronunciation you hear from people who don’t know the word.