Impossible time booking an open jaw flight

Question and a RANT.

I always book my own flights, often multi-city. Now I am so frustrated, I am about to give up. Trying to book Business Class from Lisbon-Buenos Aires, return from Rio to LIS. Totally flexible dates. I found a fare on Lufthansa I could live with, but the return is way too high. I thought I'll just go Premium Econ for the return, and they literally will not allow me to fly a different cabin for the return.

It would be cheaper to fly TAP back to LIS, but more on the BsAs leg. I can't book one-way on different airlines or the fare skyrockets. Just spent 20 minutes screwing around with Iberia Air's site and it is a mess and finally gave me nothing but error messages.

What is going on?? How can I just get this done?