Hi Ian, thanks for the wishes / suggestions!
Can you provide an example of one such pattern? Mango won't complain about the repeat modulus being greater than the periods max value generally. Syntactically it makes sense, it just doesn't add anything. I did try saving with my day field set to "NOO" and got a validation popup and not redirected from the page. I change it to "SUN" (with day of month as ?) and it saves. Was not able to reproduce.
The "Select cron pattern" section can be a bit strange. Only the resulting cron pattern input needs to be correct.