Re your query as to the validity of obtaining twin training approval: I tend to think it's a good thing to have, even if you don't plan on staying on as an Instructor for the long term. I know of quite a few GA operators (not involved in flight training) who look favourably on twin training approval - as it enables them (through your approval) to endorse new starts under you on type, instead of having to send them elsewhere. It's only one point, but a worthwhile one I think. In short, you can never be over-qualified in this industry.