Control + Space will almost always work on a new line, but yes, not if there is a serious syntax error (like a missing } or ;).
If Control + Space does not work, try pressing Control + S (Save) first, to see if there is a serious syntax error.
As far as a master list of commands, no, there is not that right now, but I hear the call, and I'll try to put something together.
There was (and is) a reason why a lot of commands are omitted. set_string is obscured because I don't think it is required for simple adventure games.
It's a slippery slope really when you start down the path of what's useful to intermediate users. I think that a lot of people here are moving way beyond the simple use-cases and into intermediate / advanced use cases.
I did have a separate version of Adventuron planned for that, without the "classroom" monikor.
I will reveal set_string to the autocomplete in the next release, but I believe "add" has always been there, from day one of classroom. I certainly added "append" to the list of revealed commands.