I didn't see a second floor. I watched both videos. I saw a little bi of egg crate near the bottom in the first video. My question is, how did you get the egg crate to with hold the weight of the rocks? More support? Different form off egg crate.
Yes, it was a concern but fully tested before use. I put it in the garden with 211KG of stuff on it and left it for 2 weeks before I put it in my tank with 100KG of ocean rock.
They key to holding the weight is keep the PVC pipe as short as possible and use as many as you can without it being ridiculous. If you want higher you can put more layers of egg crate with legs on top of the other layers, like a stack.
One thing I did find when I emptied the tank was a lot of crap can accumulate in the pvc pipes, so that is a thought.
Also, the egg crate on top is doubled up. If only 1 layer was used, it would sag badly.
Also, check it out: