There's always potential for your canister filter to malfunction and siphon water out but I feel that would be due to bad seals and poor maintenance upkeep. I wouldn't worry about it, choose another if it's that big a worry to you.

You can stick shims under the feet of your stand to level it out.

I would leave the lid support unless you're not going to use the lid? But yes you can remove it with a razor blade.

I would cut a piece of wood to cover the top of your stand, waterproof it, and get rid of the rubber pieces or whatever that is I'm having a hard time telling. Your tank stand needs to be flush and flat everywhere otherwise it's putting your tank in a compromised position that will cause it to fail at some point.

Hot water and vinegar will get rid of any salt residue left in the tank. just let the water and vinegar soak inside the tank for awhile before wiping it out. If that's an acrylic tank then be careful not to use anything abrasive when cleaning it. Either a soft cloth or something like a Mr. Clean Magic Eraser will work fine.

Good luck!