• ETA for UA is 10:20am - how long is the immigration line? Is there an expedited line for business class? Also I have an ABTC/APAC card - has anyone used one here?

Variable. No class of service expedited line, and no idea about the card. I'd expect 30 minutes, but it could be a lot more, or sometimes less. Customs line seems to flow a lot better as of the recent past.

• I gather that I should allocate 4 hours between flights for a ground transfer from EZE to AEP. I don't check luggage.

That should be fine. Absent some highway closure or big wreck, it shouldn't take an hour in the car.

• How much time to travel from downtown BA to EZE for Friday mid-morning flight to Rio? Is there a lengthy security or immigration queue?

Plan on 45 minutes. The variable here is that both lines are lightly-staffed at this hour. I've departed EZE at most hours, from early AM to midnight, and the only thing I know to expect is that I never know what I'll find until after I walk around that corner after the ticket scan pre-security. The last few times the exit passport control seems to have improved, as most Argentines can use automated gates.