Thanks. I am going to put plywood on the bottom of the stand to level the floor and for waterproofing.I would put a sheet of plywood directly under your sump. Those beautiful tera cotta tiles are not perfectly flat and that could cause the seals in the sump to fail, unlikely but a piece of plywood would prevent that.
Can you remove the screws and re seat the top frame. Another consideration is if you are going to put a plywood skin on it that will provide support as long as it is screwed into the frame that should be enough.
As far as the top, I took out the screws and it's still sitting like that. I guess the wood glue dried because I can't pry the top off. I'm going to skin the top with plywood too so I think it will be ok. I just can't figure it out. Everything was cut, measured and leveled perfectly. It's a mystery to me. Lol.