What was the hb at your scan? I'm pretty sure that being able to see a hb on an abdominal ultrasound vs a vaginal is more reassuring re miscarriage rates in early preg. Im not sure why but have read it a few times. Brown mucous was probably left over from implantation.
I wouldn't worry about scan being behind re dates. I'm having Id twins and they measured 3 or 4 days different from each other at dating scan