In May, I need to travel to Vancouver and then to Sao Paulo for work, and I would like to buy a separate ticket and stop in Rio for a couple of days of vacation on the way back. The budget only allows me to buy a coach ticket. My schedule is tight and I have limited flight options, so I would like to book soon.

The fare is $4032 for this itinerary:
SEA-YVR May 2 UA 8396 (Operated by Jazz)
YVR-GRU May 3 UA 298/843 via ORD
GIG-SEA May 7 UA 128/CO 1568 via IAH
On united.com, it will only book a Z business class fare for flight 843.

For this itinerary, the fare is only $1547 (all coach):
SEA-YVR May 2 UA 8396 (Operated by Jazz)
YVR-GRU May 3 UA 298/843 via ORD (coach)
GRU-SEA May 7 UA 842/635 via ORD

For this, it is $1554 (coach):
SEA-YVR May 2 UA 8396 (Operated by Jazz)
YVR-GIG May 3 UA 298/843/861 via ORD/GRU
GIG-SEA May 7 UA 128/CO 1568 via IAH

I called, and the CSR gave me the same prices for these routes, BUT he said that UA 843 is a coach fare, not Z. He told me that the lowest fares are not available with both a stopover and an open jaw. However, he couldn't explain why the fare difference is so high or why the website states that I have to book business class on UA 843 if I want itinerary #1.

Stranger still, in early December I looked at itinerary #1 on united.com, but I remember the price to be less than $1600. I might be mistaken, however, and I did not print any evidence of the fare. Another odd thing is that I checked all three itineraries on December 24, and all three routings showed availability in business class only, with prices in the $6000-8000 range.

Sao Paulo to Rio is only $70 each way on TAM, so I might take itinerary #2 for the $1547 fare and buy a roundtrip on TAM. I am concerned about the return flight if I have to connect at GRU from TAM to UA. I don't want to worry about a missed connection or checking bags on different PNRs.

My questions:
1. Why won't united.com let me book the coach ticket on 843?
2. Even if I book coach on the phone, the fare is over $4000. Why would that be? Is this a standard rule for stopover/open jaw combinations?
3. Which Rio airport is best for a short trip from Sao Paulo?
4. Should I be worried about connecting at GRU on the way back? How much time should I allow for the connection when I have different PNRs?

Thanks for reading this long post. I'd appreciate any advice or insight.