1. “Different variant” means the -200, so we have to be looking at Aspen Air prior to the Air Wis (ZW) merger (What airport are you referring to specifically? Aspen?)

On the assumption you are referring to Aspen, we are looking at Aspen Airways (The World's Fastest Chairlift) operating on behalf of United Express, which means the aircraft variant was a -100. Here's the flight:

United Express UA 3780 Los Angeles (LAX) 700a-1005a Aspen (ASE) BAe-146-100 Daily X3

2. Vail / Eagle (EGE) — I’ll offer that this was the ZW 146-300

Vail/ Eagle is one of the airports we're looking for, but it was not served by United or any of its affiliates.

3. Only two other AA narrow-body options here (Steamboat Springs), 727-200 and (more likely) MD-80

It was not the MD-80. In fact, outside of Denver, Colorado Springs and Grand Junction, the MD80 was rarely seen at the mountain airports serving Colorado ski areas. I believe AA once flew one into Durango for awhile) Perhaps those little wings didn't provide enough lift for the higher elevations? In any event, that leaves the 727-200. Here's the schedule:

American AA 1392 Los Angeles (LAX) 930a-1237p Steamboat Springs (HDN) 727-200 Daily
Breakfast served enroute