It is frustrating and, yes, buying multi-city and multi-cabin tickets is very tricky. Google flights will do you multi-city very nicely but won't do multiple cabins and aa.com will let you do multiple cabins but it doesn't do multi-city or partner flights very well at all, though you can book them if you can find them on google (as you found out).
I'm not sure you could have done much better here - I would suggest that you should have put it on hold and called in for PEY, but with partner flights the hold option wouldn't be there. So you're left with making the changes within the first 24 hours so that you cancel for free if it doesn't work.
Also, you really have no idea what the fare might be for what you want, since neither google nor aa.com will quote you a price for it.