One of the company involved in this year program also actually think to be organized as "Benefit Corporation". They are from Italy and as I understood in Italy this is possible (one of the very few countries) and their business idea is strongly socially oriented. I didn't ask myself if this is legally possible - good question. I guess that you would have to discuss it with some legal person and as Rockstart is partially financed from Dutch lottery (which is by definition charity) maybe they will agree (also to be honest many of ppl around Rockstart are already proven entrepreneurs and they dont look for money, they really like idea of helping star-ups and to be involved in this community). So, your main question needs to be discussed.
However, in later stage when you actually need investment to scale it might be more difficult to raise money (from simple reason that investor ask for profit - this is truth). It may need some other means to raise money for your company. There are some social funds (on London stock exchange as I understood) in UK where there is money for such a companies and one of our mentor has company that they look for money in this particular fond.
Between priority 1 and 3 there is little difference. If you are able to make business that has no losses, you might do some profit as well (and reinvest it in your idea again of course).