Certainly, you’re right that reading it as “konnichi” is very rare. It’s rare enough that I only sometimes misread it that way when it appears in the news or similar contexts. So I think that was a good decision.

By the way, I tried testing it further while thinking back to the kinds of tests I had when I was in school. The accuracy is still excellent. My guess is that “一日” and “分別” are being handled in a similar way to “今日.” “分別” is very rare, but I don’t think “一日” is all that uncommon.