Don't worry, after your friend cuts off those legs the bottom frame will distribute weight more evenly across the tiles so they shouldn't crack. 2 sets of supports in the middle is perfectly fine for a 180, and most store bought stands are made from 1x3's so 2x4's are more than enough (and with 3 in each corner they'll be closer to using 4x4s anyways). If u want to be extra safe while you're at it u could add diagonal supports, but its not really necessary with adequate cross bracing enclosing each set of vertical supports. Just try not to stress out about it, the only way the current design will fail before tomorrow is if there was a major earthquake or someone tackles it from the side, and once u make those modifications it will be just as overbuilt as every other joey style stand 
