I think -I/--ignore-assertions is the right solution in this situation.
If you did want to check assertions before (and while ?) running hledger add, I think you can do it by mentioning all files, with the one you want to add to listed first:
hledger add -f creditcard.journal checking.journal savings.journal ...
(not -f creditcard.journal -f master.journal, which would read creditcard.journal rwice).