No. It will dissolve in the water.
Make hills using “egg crate” - cut the pieces and layer them like a pyramid. I use zip ties to hold all the pieces together. 4Pcs Aquarium Fish Tank Bottom Isolation, Black Grid Divider Tray Egg Crate Louvre for Mixed Breeding https://smile.amazon.com/dp/B07WRMC786/ref=cm_sw_r_cp_api_i_6FR8ZG3NZNAPTEXGV9W3
I’ve never had a fluval so I can’t comment on the cleaning process but the cascades are very simple. The impeller comes out easily for cleaning. I’d imagine the fluval is the same.
The basking platform can be made with egg crate and zip ties.
It depends. I’d say use egg crate thingies if you have pointy rocks and not a lot of substrate. If your substrate is thick enough and the rocks dont touch the the bottom of the glass at a weird angle i don’t think it’s necessary. Also depends on aquarium size. A 5 gallon with like 1-2 rocks is less problematic than a lets say 20 gallon rimless with a ton of rocks, in my opinion. Here’s what I found on amazon: Pomeat 4Pcs Aquarium Fish Tank Bottom Isolation, Black Grid Divider Tray Egg Crate Louvre for Mixed Breeding https://www.amazon.com/dp/B07WRMC786/ref=cm_sw_r_cp_api_i_eMjjFb425DPB5
if i may make a suggestion, friends have used these to great execution. you can get em all over and build em inmultiple , a friend made a multi bay one out of these for the 00 gundam models.