Hi Mumofthree,
DD was always been super quiet at preschool and socially with adults she doesn't know. I too used to feel frustrated by her lack of manners with others when she has such beautiful manners at home - when in reality I think dd just gets a bit overwhelmed (sorry - I don't mean to dismiss your concerns about selective mutism, certainly follow that up - I just mean that I understand that frustration).
I would encourage starting school if you think she's ready. We've had some issues this year with starting school that we're still in the process of addressing but there are a couple of thoughts that have kept coming back to me - 1 So glad preschool is over. Dd didn't hate it, but it was never a good fit. She was quiet and independant in a way that she never is at home (though - if I look for a silver lining it was nice to know she could be independant as she's like my shadow at home!) 2 I wish, wish, wish early entry rather than starting in grade one had been available to us. Because of dd's birthday it was just never going to happen but i think it would have made a significant difference to her confidence - and it would have got on top of some of the social issues as she would have been with peers.
It may or may not pan out for you like this, but dd, now 5.3 is much more socially confident than she was even 2 months ago. I'm not sure if it's just a maturity thing or a combination of maturity, school, etc. But she was always such a little square peg at preschool she never got the chance to just be her.
It's great that the principal is saying yes to grade one next year. I am now about to contradict what I just said above - I do wonder if rather than focusing on getting her in to school a few weeks early, if you'd be better off pushing that she go in to one of the existing classes next term (rather than the late starter class) on the basis it will allow for a smoother amd more emotionally stable transition to grade one (and getting in writing - if you haven't already - that she will go in to grade one next year). That would allow for a smooth transition and you could make sure the school was aware you were making a concession and were therefore very flexible and cooperative
But if you think she needs to be there now, push for now.
Good luck - and keep in mind there are no perfect answers in the real world for our kids. Because what you're looking at doing is different to the norm it's only natural to question whether you're doing the right thing. You don't have a useful base for comparison. Go with your gut