No, just boots. It's a popular trail.

Only when there's substantial low elevation snow (such as last week - but I would bet it's all melted now). Diablo Lake is only about 1300 feet above sea level, so it doesn't get nearly as much snow as you might think, and even when there is snow it's still plowed year round up to the Ross Dam trailhead so Seattle City Light can still access the dams. I've been up there when there was 2-ish feet of snow in the Diablo Lake overlook parking lot (and gated closed of course), but the main highway itself was totally clear and dry.

Pro-tip: you don't need to wait until summer to go to Diablo. The road is open year round to the Ross Dam Trailhead, and there will be far fewer people up there before it gets warmer.

Elk more likely, they're much closer to Moose in size and there are lots of them around Packwood.

Too much water to freeze over, but it does grow a pretty gnarly wall of icicles behind and around it when it gets cold enough.

Whoever downvoted you clearly has no idea what they're talking about. This is abjectly true. Moose will absolutely fuck you up if you get too up in their business.

You can't get to the riverbed anymore, no. There's a fenced off viewing deck at the bottom now.

The fire was up by the Horse Camp, several miles north of the falls.

1994 and 2014 were the two bad fires in Tumwater Canyon.

The light blue color in this case comes from air bubbles, not silt. This river isn't sourced in a glacier at all. The light blue stripe below the waterfall is a former lava tube, so it's quite deep in that area. As the water gets agitated where it drops into the crevasse at the waterfall the air bubbles end up getting pushed into the deep water where the current is strong, and they get pulled downstream rather than bubbling to the surface. If you turn around on the bridge this picture was taken from, you can see where the bubble trail eventually works its way to the surface and the light blue color fades out. If it was caused by silt, you'd see the light blue color everywhere in the river, not just in the deep sections, and it would be less blue and more milky on overcast days like depicted in OP's picture.

It was about 12 miles northeast of Snoqualmie and 20 miles northwest from the pass.

Are you thinking of the Loch Katrine fire, which was in the North Fork? About 18-20 miles from the pass. There were no fires in the Middle Fork this year.

There was no fire here.

Keep in mind that the Snoqualmie River may have a large drainage area, but its a relatively low elevation basin with almost no glacial ice serving as its sources - the Overcoat Glacier is the only one of significance, and it's not very significant in the first place, and only about half of it flows into the Snoqualmie (the other half goes east into the Cle Elum River).

There are a couple tiny glacial remnants (basically stagnant but permanent ice) around too, but the rest of the Snoqualmie drainage sources in lakes or runoff, so when its been as dry for as long as it was this summer, it will of course suffer.

It's not really a good bellweather for the health of the rivers in the Cascades though, since other than the Skykomish and Stilliguamish, all of the other major rivers on the west side are fed by a ton of glaciers, and will be plenty healthy for a long time still.

The vast majority of the volume of the river is diverted into the hydroelectric system for the majority of the year. Yes the river is low, but the falls look like OP's picture very regularly. PSE is required to allow an minimum volume of water pass over the falls regardless of how low the river is, it only looks like your picture when the volume in the river exceeds the capacity of the power stations (which generally occurs on and off for about 7 months out of the year).

Even before the power plants were built, it looked a lot closer to OP's picture than yours during the dry season.

It's plenty studied, but definitely very under-monitored.