They make a leveling compound for under tiles, but I don;t recall any name brands or product names…I’d ask your friendly Home Depot of home supply store specialist.
As for as how to keep envirotex from spilling, you can use that blue painters tape to make a small wall around the perimeter to keep etex from dripping onto the floor. It’s always a good idea to do a small project with envirotex to see how long it takes to setup. It usually takes over a month to get rock hard.