Bless, Ele, you guys don't seem to understand how beginners tend to be. It's not like you can go into a room and say "hey guys, I'm here to teach you the game, now do as I show". Around 70% of beginners are not even participating in the chat, and the ones that do tend to just comment on their or other players' plays.
Where am I going with this? The matter of a fact is that if a person doesn't enjoy the game, a guy that sits in the room with him won't change a thing. If a beginner enjoys the game he will go beyond the game and watch youtube tutorials, visit forums, etc. You seem to make really big deal that the new user group would lower retention rates but that's not how it works, people that enjoy the game will stay and people that don't enjoy it will leave.

Whether it would be HS or TA there is a limit of what you can do in public beginner room. You can't just teach them the game itself by going on a fight and saying "watch what I do". You have to really understand this: beginners don't try to learn the game, they are trying out the game. No beginner joins the game and the first thing they think is "wow I really want to be good at this game". No. They are trying out the game and seeing if they like it, you cannot simply lower the retention by coming up to a group newbies and saying "I'm here to teach you the game so stay".

TAs or HS are only there to answer general questions, give various suggestions, link them to threads that can explain them about the game and maybe get rid of some alts. Teaching in such conditions is simply impossible and unnecessary.