OMG! She should be crawling up your arse trying to build bridges with you if she is petty enough to not speak to you for saying that you have chosen not to baptise your child, not 'settling out the ground rules' for your relationship! She sounds like a treat!
In regards to the parneting advice, id probably want to respond with something like 'i dont want your parenting advice, i live with one of your children and i cant say im too impressed at the moment with some of his qualities, and especially the absence of his spine. BUT DONT! ha ha. No, i think that this needs to be responded to by your OH on behalf of both of you, or by both of you together, in person. Putting something down on paper/email will never be taken in the manner/tone that it was intended, but dont just ignore it completely.
I think that you should put your foot down with DP over not wanting to get involved. He cannot bury his head in the sand over this, especially if he wants everyone to get on and have his quiet life. I suggest sitting down and going through the email together and discussing whether he feels any of it is reasonable/unreasonable, telling him it isnt acceptableat all for his mother to act in this way and that you will not have the welfare of your child dictated to by anyone. Make sure he understands why this type of behaviour needs to be nipped in the bud, by him, and immediately.
If you cant get him to see how unreasonable his mother is being, and get him to cut the apron strings, then im afraid that this problem will only get worse as time goes on.