1.16.200.56 Beta Has the same issue as mentioned. Except in this case the crash to desktop only occurs when attempting to type a ~ in place as of the z coordinate. Fill exhibits the same behavior as described, except only with ~. If the z coordinate is specified with a number instead, the behavior is not present, and the problem does not occur with any of the other coordinates.
As far as the spider spawning prevention, spiders if the buttons and carpets are not placed every other block, spiders will still spawn. Buttons however, do work just as well as carpet if used every other block.
1.16.200.56 Beta Has the same issue as mentioned. Except in this case the crash to desktop only occurs when attempting to type a ~ in place as of the z coordinate. Fill exhibits the same behavior as described, except only with ~. If the z coordinate is specified with a number instead, the behavior is not present, and the problem does not occur with any of the other coordinates.