Is the fact that C# produced executables also a problem?

Trust me, the conversation never even gets that far.

just not installing the python runtime on them

We weren't! We were using repl.it (or something very similar). I don't know what the story was at other schools, other than many other teachers also wanted C# but had to do Python (it was when I came across this that I finally accepted defeat in trying to get another language in instead of Python. I wanted to start with Pascal and then do C#. In the end I had to do HTML and Python. i.e. the status quo).

Generally agree with you that teachers should be able to choose at least one of the languages to teach.

We're supposed to be able to choose both languages, but school admins are taking away one of our choices.

if it includes JavaScript?

I wouldn't do that at the same time as HTML - maybe later, separately. As I've said, as teachers we only teach one concept at a time.