I definitely learned my lesson... I'm a novice international traveler. But who knew codeshare flights had seat reservation issues... codeshare is not a new concept. Iberia's software is just plain broken. If Iberia software cannot handle codeshare seat reservations, then their software should *not* allow me to pick seats + accept and charge my credit card for those seats + immediately tell me there's a balance due for those seats even though the seats charge was instantly completed through my credit card company. Note I was able to pick my seats on AA's flight leg no issues, so AA's software does correctly handle codeshare flights.

Iberia must be well aware of this bug due to many other customers like me hitting it, but when I talk to Iberia, they act like it's the first time they heard of the issue.

Re "you have a perfectly valid ticket to fly to Spain".... legally you may be correct, but any family booking a long duration flight expects to be able to pick seats in advance. I didn't see any notice during my booking process saying I would not be able to do that.

I didn't not see any non-codeshare flight that didn't require an overnight stay somewhere... it's possible I missed some. Alternatively I could have booking each flight leg direct with each operating airline, but I assumed (maybe incorrectly) it would be more expensive, and increase chances of lost baggage and missed connections vs. booking via a single airline who codeshares to other operating airlines.