"Attempting to discern "unwritten house rules of etiquette" is too much to ask of newcomers to the music"

No. Well, yes, but that's not what is being asked, I think.
There is a difference between common sense sensistivity and having to discover arcane rules.

Also, all sessions are not the same.
Sometimes I have taken part is a session and won't go again, not because they were unfriendly, but simply because it wasn't the sort of session I want to be in. I wouldn't dream of interfering with the way it was run. In particular, there is a need for very open sessions where anything goes and all newcomers are welcome, and the musical standard may not be consistently high, and there is a need for better and more experienced singers or players to get together and enjoy listening to each other's contributions, and there is no need for anyone to insist on trying to turn one into the other.

The idea of 'slow' Irish sessions in not new - here in Cambridge there used to be at least three different grades of sessions at different places and times, and you soon found out where you fitted in best.

Anahata