You can argue both ways. You sit on a chair and you sit on a toilet. Most chairs arent made out of porcelain, but you certainly could make one.
A toilet however serves a different purpose than a chair. This is the main definition that matters, because you can sit on a table or a roof, but those items are definitely not chairs.
You could sit on the floor. That is definitely not a chair.
I don't see why not. The "toilets" in old-style camp outhouses are often called benches, and indeed they are pretty much benches with strategic holes in them.
Definitely. It's where you sit when talking to someone in the shower.