(2) on what basis are we assuming the Earth is flat?
Re-read the OP and realised you'd already given an answer to this
Firstly we live on a flat earth, don't believe me go and look at the ocean. It is flat as far as the eye can see and then still flat after that. look into the laws on fluid dynamics
So I'll answer in regards to this ^
go and look at the ocean. It is flat as far as the eye can see and then still flat after that.
Well, the Earth is quite large. Lets compare the amount we can see of the sea to what an ant can see of their world. If you were as small as the ant, everything would look a lot flatter wouldn't it? Because not only would you be closer to the ground limiting your view, but it'd be that much bigger in comparison to your size
you can even test this just by sitting in a car seat. Raise the seat as high as you can, lower it as low as you can. Look at how your perspective of vision changes on the road.
A camera can demonstrate this even better. This is the surface of a Tennis Ball:
Looks pretty flat doesn't it? Yet we know a tennis ball is a sphere
The Ocean looks flat purely because of how vastly big it is in comparison to our own size. Not because of its shape
look into the laws on fluid dynamics
Well, this is fluid dynamics:
Everything around us is in 3D. We can even prove the moon is round by the patterns on its surface. There's 16 unique patterns visible and it varies which you see depending on the country you're in. Australia sees a rabbit.
So if even the moon is a globe, if everything around us is in 3D, one would then have to wonder why we'd be living on a 2D plane of existence surrounded by 3D
Another aspect is anything when scaled up worked differently in the extremes from what you'd expect in a smaller amount. Laws of dynamics for example reverse the controls in a airplane once you reach near the sound barrier.
Water tension for another example. It's harmless if you're touching it from up close. But slam into it and it'll hurt. It can even break your ribcage if you drop into it from too great of a distance or from the wrong angle.
And what about when it rains? Look at a leaf! The water clings to the leaves and you can even move that leaf slowly before the water will fall off. It can even be practically upside down before it drops. But shake it and it'll come off. This is in line with what the spoiler above explains about the mass that it's clinging to and the speed involved.