Yes a lot of questions!
I reply with two hats on; as one that runs a green building company & the other as someone in architectural practice. So will tackle all of your questions. Others may have other specific advice but here's mine for what it's worth-take of leave it.
1) No experience needed to get a job with any builder if you are prepared to go as a builders labourer, just brawn. This will give you an insight into building generally, but as you state, you have some experience. Can't beat lots of site experience with a good building company who have a variety of interesting jobs on the go (even better if green). If you can work towards gaining a trade-even better. I did joinery at night school for 2 years, coupled with working on site, which gave me the trade and made me more employable. Very few unqualified people working on a trade are allowed to work on site now, other than builder's labourers. So get a trade-min of 2 years training. As an employer, I would be looking for someone with, or willing to work towards having the basic conventional building trade skills as not all jobs will be green ones. The reality is that green building skills come with experience. Short courses will give you a grounding or insight but that's all, so don't think there's a quick way of learning a skill. You have answered the question your self. Getting a job with a non green builder is a start but even better with someone that is working on green projects.
2) We set up our building company as qualified trades people/professionals and then got into green building by learning & experimenting over a period of time. One still has to do a decent job, get paid & earn a living, so be good at what you do first before you try the experimental stuff. We've been in business for 18 years and we are still learning both conventional & green building skills. Our apprentices all attend college to learn their trade and pick up the green skills on site or on courses we send them on.
3)Making a green living; Yes, but in reality one needs a wealth of experience to go on your own in business. Our business can not rely solely on green building work but we try to influence all our projects when we can, educating other professionals and working with clients to build more sustainable buildings. If you want to go into building speculative green housing there's a whole lot of skills you'll need other than basic building. Walk before you can run!
4) Economies of scale: that's why green building materials tend to be more expensive but not all ways. Things are getting better as green building materials are becoming more accessible and affordable. It would be pointless tendering/pricing for a job using green building materials when your competitors are not, you'll price your self out of the job. We tender for conventional jobs and once we've secured them then we work on the clients or architects. As we are known as green builders we do get asked to work on green jobs & we do offer a design & build package if needed, only because we have the in-house professional skills to do so. It takes a long time to build up a good reputation but short time to loose it! Green building is viable now if the client wants the produce. Market forces apply.
5) Planning & building Reg's can take a little longer. Planning should not take any longer than conventional design. There is occasionally opposition to new materials/techniques but on the whole a clear thought through case made should win objectors round. B Reg's can be a little problematic-I've had to spend a lot of time on some jobs explaining how straw bale buildings, impact foundations, or composting toilets work etc. I don't find it takes the fun out of it, as you're spreading the word. Yes it can be frustrating some times, coming up against people who refuse to learn or change their opinions but that's the challenge before most of us.
6) Both the design process and the building process is incredibly rewarding. Doing as green as you can is immensely satisfying with it's unique feel good factor attached-plus it's fun. Can't beat it.
If you are really interested in learning green building skills then go for it, get your experience and make it happen.
Good luck Zac.