Thanks for your patience, Vito
Well, I got it to work, sort of... I think the upstairs light is wired exactly as you say, and it worked pretty well (as mentioned finicky), until I set up my 2nd 3-way switch, more on this later
So, the 2nd 3-way switch I did the following (maybe too clever for my own good here....haha):
1. put masterswitch downstairs
a) hooked GE black into the blue wiring in the box
b) hooked GE blue into brown(1)
c) hooked GE white into brown(2)
d) hooked GE yellow into brown(3)
2. put aux switch upstairs
a) hooked GE red/white into blue wiring in that box
b) hooked GE yellow into brown(3) (from downstairs - positive about this one)
3. finished masterswitch connections in upstairs box
a) hooked brown(1) into orange+yellow for downstairs lights (turns out orange is one row of lights, yellow is another)
b) hooked brown(2) into white really going to upstairs light(in order to get neutral - my only place to find neutral)
The 2 most obvious questions/issues;
a) unable to verify if I hooked brown1 into brown2 and vice versa, as the previous switch had the cables bundled. What happens if I mixed them up?
b) can I use any white/neutral? seems like it should work?
Result - Masterswitch for upstairs lights work perfectly. Masterswitch for downstair lights work perfectly. NONE of the aux switches work at all....
I have verified that brown1 goes to brown1 etc. When I switched them, the main electric panel started humming, so I promptly switched them back
Also, I have found that the upstairs light occasionally will respond to a command from aux to turn off or on, but it is incredibly random, and I may need to use master or remote controller to turn on/off before it works once or twice. The other aux is completely dead..