This is actually one of the more interesting questions in space exploration in general. It has been ever since someone tried charging NASA for parking on the section of the moon they claim ownership of.

Defense would actually be quite easy. Just undesirable. There are working antisat weapons in the major powers' arsenals. However, they would produce nasty debris clouds.

And then there is also the vulnerability of the lauch site and control center...

And it is probably significantly cheaper to take down a sat than putting it up. So it would be a losing battle.

Not in general. A kinetic striker would require as much energy to reach the satellite as the satellite used to get there. Something fancier, like a targeted laser, to my knowledge doesn't exist with anything like the wattage needed to punch through atmo and still have effective kill on anything that far out (assuming you could solve the targeting problem to resolve the beam that tightly in the first place).

The satellite is in orbit. It requires significantly more energy to put something in orbit than merely reachin that altitude. Satellite killers are relatively small rockets that can use a small supersonic fighter jet as first stage, while it requires a much bigger rocket to put a satellite in space (it would depend on the orbit of course).

It is true that spacex can put a significant amount of satllites in orbit witha single launch though, so it probably evens it out.