Brakes need a transference of material, i.e. 5 or 6 hard braking efforts in a row from 60/70 mph to 10mph, then a couple mile cruise to cool them down, it's normal procedure.
Creeping is normal yes.
The rattle is most likely rear pads rattling in the caliper, this can be very loud like all the undertrays are loose, but is common and normal.
Have you removed the secondary cats yet and replaced with a Y pipe? Main cats is a big job in comparison.