I wouldn't be worried about the curb as much as how low the screws are inside the pan, as the top of the curb isn't going to hold water like the pan will. The pan needs to be watertight to 2" above the curb by code where I am. If they have screws below that, the liner should be replaced and whoever put the board up should pay for it.

Just to confirm: the concrete under the liner isn't flat, right?

To be fair they constantly mention wood glue throughout those instructions. One without the other may not work half so well.

I've found they take on water very easily if you keep them in a cooler with ice/water. I returned mine. Also seems like that type of plastic would crack very easily.

I use the plastic Snapware from Costco, but they have some glass ones too.