"I wonder why all the Summer children are gone now. Rachel's in space, Nate is in prison and Cable's dead. What are the writers doing? "
Trying to undo/ignore the 90's?
As much as I enjoy all of those characters, the Summers kids are probably the most convoluted mess of a family in comics. I can't really blame the writers for putting them all on the shelf for a bit lol. Try explaining who each of those characters is to one of your uninitiated friends...you will lose that friend. Literally, they won't be your friend any more after listening to you regurgitate that kind of nonsense.
"Are Daxamites as vulnerable to Magic like Kryptonians? "
Yes. Pretty much anyone without a magical background or origin (unless they are in possession of some type of magical artifact or have been provided with shielding from some other magic user) is vulnerable to magic. It's the anti-science.