I dont work in Maternity so can't answer from a professional stand point, but here in Australia it is used all the time during labour, dont have to sign anything to use it either. It's often our first go to for pain relief substances and can be used during any stage of labour.. I have used it with all 3 of my labours, the effects only last for a few minutes, as soon as you start breathing normal air it wears off very quickly. It does not have any effect on the baby, it wears off too quick.. You start breathing it in at the begining of the contraction and stop when the pain is gone, if at any point the woman experiences unpleasant side effects like nausea, she can stop using it and the effects will be gone in a few minutes.. I have used the gas without a midwife in the room, so many times, they check in frequently and arent far away though.. If the woman is in control of the mouthpiece and taught how to use it properly, they are very unlikely to overdose on it..
In term of family using it, my husband didnt want to, but many do take a little go to see what it's like, as I said, it only lasts a few minutes and they arent going to try much more than once because you would have a hard time taking it away from that labouring woman and they wont really try it with the midwife in the room..