It is very accessible. There are flat roads and flat paths leading right to the lake and majestic views. Day use parking area is mere feet from lake. Bathrooms are the only real issue - they are more or less just outhouses.

Not that I know of, especially in the winter time. They close the main gates there (at Trillium) in the winter, so even when you drive yourself, you have to park beyond the gates and then snowshoe in. The road beyond the gate is usually buried in snow, and the lake will also be frozen and covered in snow. If you have transportation and don’t mind hiking in, it makes for a great winter snowshoe loop :)

I am sorry. I don't have an answer. I suggest you contact the closes C of C or Visitor information for your answer.

You should contact the Forest Service regarding reserving the amphitheater, or discuss with them such a special use of a public locale without impeding use by other citizens. Relative info can be found by entering “us forest service, trillium lake” in a search engine (this site won’t allow my entering links or I would). tel:(503)668-1700

We had no problem with Verizon.

Yes. You will share the lake with non-motorized devices, maybe kids and swimmers near shore, fishermen around and on the lake and the handicapped dock, and Mt. Hood dominating. The lake is not large and I have never seen rough water on it, but stormy weather is predicted so beware. It is a popular place and unfortunately can get too busy for my liking these days, but then I remember it 40+ years ago.

Yes, especially after the recent spate of unusually warm weather I’d expect the road into the lake to be navigable.