This is just a spacer to raise your grate so it will be flush with the floor.
take a look at the Ebbe E4400 drain riser.
No this will raise your drain which will make it harder for the water to drain
Sorry Carl. This item is for round drains.
Yes, this product can be stacked.
It means that the attachments screw holes measure 3,3/8" on center. If the screw hole distance across the pipe in your floor flange measure 3,3/8" on center, this drain height extension kit will fit.
3,3/8" on center