I'm in Ontario and you can get an NT scan at any age. Also, they can tell you the sex of the baby here. I don't see why not. It's just another body part. I am interested to know what province you are from that has these restrictions.
I think a cake with a few close family members is fine and fun. You've waited this long, a few more days won't hurt. After your appt take your envelope from the midwife right to the bakery and then you won't be tempted to open it.
I personally will be finding out with just my H when the baby is born, but there is nothing wrong with what you want to do.