Gas heater lights then goes out

The thermopile in your fireplace puts out millivolts, nothing near the 120V the light switch was designed for. It's probably just a matter of finding a switch with a low enough on resistance. A generic low voltage switch from a electronics store, or ripped out of a toy, would probably do it (for example a 12V SPST). Really here the smaller the better, but beware of anything too cheap as that may put you down the wrong path again.

There are some fireplace specific forums which could help, particularly with a model number given. An example is http://www.hearth.com/talk/ . The above image is from "Discover Circuits Magazine (December 2009 Issue 4)", which has a guide on this exact topic along with a clever booster solution from one "Mr. Wily Widget". Find it over at http://www.discovercircuits.com/dc-mag/Issue_4/pg-7.htm

You can also try cleaning the thermopile which may have crust of soot. Measure the voltage between those two exposed wires to see if your cleaning makes an improvement.

It is (very) surprising that the gas company (Xcel) says "everything's fine." This document from a different gas company certainly calls out non-blue flames as an issue that needs to be resolved. As does this site. And Xcel themselves.

Happening on all appliances does seem a bit less likely to be "and suddenly the air adjustments on all three went whacky" than a contamination issue - possibly water in the gas line?

I wonder how your furnace tech visit went, and what your neighbor's flames look like.

Editing post-comments: Ultrasonic humidifiers make small water droplets (without evaporating the water first) and then those small droplets evaporate while floating around - if the water contains salts or minerals, any salts or minerals in the water will end up floating around the house as tiny specks when the water evaporates from the tiny droplets. Those, evidently, are what is contaminating your gas flames as they are sucked in along with the air required to burn the gas. Old-style "sponge" dehumdifiers (should) only result in water vapor in the air, as they actually evaporate the water (and this eventually leads to a build-up of scale and salts in the humidifier and its sponge element, as those do not evaporate.)