Earth's bodies of water are indeed mostly level, which in a spherical frame of reference means the surface is the same distance from the centre of the Earth wherever you are. But if you want to try to flatten the world with definitions and etymology, it bears mentioning that the latin for "level" is "libella".
Air has weight, the pressure of air at ground level is from the weight of all the air above it. A gas that has weight can only expand until its internal pressure and its weight are in equilibrium. Even if you pretend gravity isn't real and why anything weighs anything is a complete mystery, there's still no reason to expect the atmosphere to fly off into space.
What two opposing systems, what the hell are you talking about?
Sure, yeah, there's no proof the Earth rotates except for all the proof you've decided doesn't count or isn't real. Or the evidence that you pretend has a better flat-Earther explanation than the real explanation.
Same response as 4.
Again, same response as 4.
And, weirdly, the actual linked video doesn't even mention any of those numbered points, it's got an even stupider argument, "these two numbers have a relationship with each other and add to something interesting, therefore Earth is fake". Like, at least denying evidence bears some resemblance to a logical argument, this is pathetic.