I honestly don't know if cards can be added to a checklist later. There must be a way, since some sets come in 2 series, and we don't wait for Series 2 to release before we add the Series 1 checklists. I don't know how to do it though. I haven't added a checklist before that I didn't believe to be complete. I do believe we have the option of putting "TBD" in the place of a name if a card is known to exist but can't be identified. Then it only requires a name change to fix later if somebody has the card. I just don't know about adding a set if you don't know how many cards are in it.
I wonder if that mystery card 25 is actually an unnumbered checklist, or some kind of advertising or coupon, that would end up being added as a single card insert anyway.
I'm going to reevaluate how I collect after the new year. It's just getting way too expensive for the new stuff. Sometimes I just want to buy a pack, not a whole box or even blaster.