Sounds like your making a little progress.
Let me state too that I'm not a "plumber". I'm a ships engineer so I'm into everything from diesels, refrigeration,a/c, steam, hydraulics, electricity. A jack of all trades expert of none type.
Boy if there is not venting screw anywhere on the upper zone it is hard to get out. If it is possible to isolate the lower zone(s) and you have some access vavles (garden hose fittings) before and after the boiler it is possible to take water from the street and pressurize the upstairs and try to get enough "flow" to blow the air bubble down to the boiler and out into the sink with another section of hose. Hard to explain in writing.
If you have any work done by a plumber may be a good investment to solder something in upstairs to vent the system.
Not sure about your thermo not firing the boiler till 84'. If the thermo is new and the simple round mercury type I'm sure it is ok. Depending on how new your system is and its controller. Sometimes the boiler fires independantly of the zone pump. For example my system is 50 years old but the downstairs zone starts firing imediately when the zone thermo calls for heat and the pump comes on. When the upstairs zone calls for heat the 'pump only' comes on and circs hot water....when the boiler gets "cold" the thermostat at the boiler closes and fires independant on what the zone thermo says.
I'd take another look at your baseboard upstairs to see if you missed the vent. You still are air locked up there it is just hard getting it out.
Bet it is the last time you hire that guy who did your work. He can't see it from his house,...jerk...Too bad you didn't sive closer, I shoot over and try to help you out....For a few plugs of course
Good Luck, Jon