PhilB,
It seems to me that you have done your research and have some specific questions, unlike many.
#1 Yes it is easy to get resin to float with microbubbles alone.
#2 Try to start with 50% MB's in each measure of part A and part B of resin, adjust accordingly.
#3 MB's do tend to suspend in the resin, but be sure to mix both parts before mixing together.
#4 It would be my advice to mold in line tie and hook hanger attachments in your two part mold.
Good luck and have fun,
Douglas