You set the stall on heaps of mortar so the mortar will squish out when the stall is set down. The mortar will contact the under side of the stall in multiple places and prevent the floor from sagging under weight. It does not have to be a solid covering. The drain positioning is critical. There are fairly easy ways to make the connection once the drain matches the hole in the stall floor. A good plumbing shop can provide you with be best drain to use. The drain is attached to the trap after setting the stall in place. The drain needs to be 2" and be trapped and vented. It may be necessary to break out some concrete. Fortunately, you do not have to do a fancy job of concrete finishing after the plumbing is installed. You are correct, you can not access the drain after installation. You may want to consider a professional to do the drain plumbing and stall installation. You pretty much get just one shot at it, because it will be cast in concrete.