1/ I think you should glue them down with wood glue directly to the bar top. Why take a chance of them floating within the E while it sets. But, thinking of it it now that might look cool coasters at slightly different levels ?
2/ Just be sure to seal the cracks or seems on the underside with tape etc… to stop possible drips from coming thru. If you choose to take top off and seal. Be sure to keep the top ferfectly level so the finish is also level when placed back on the bar.
3/ I split the pours in two. I think it said that you were only limited to how thick each coat can be ?
4/ Use a propane torch to remove air bubbles before the coat cures. The hair dryer method did not work as well for me. As the instructions say ventilate the area. The more I think of it maybe placing some coasters under the sealer coat and some above it may really look cool ? Might give the top a feeling of depth when looking at it ?