Kona stain and Varathane

I am refinishing a vanity. I stained the top with Kona and am using varathane satin poly. The first time I did it, I made the mistake of lightly sanding between coats. After 4 coats the surface scratched quite easily and straight through to the wood. I had to sand the piece down to bare wood again and start over. The second time my stain did not go on as smoothly. The finish came out much darker losing much of the grain. I did 5 coats of Varathane (with NO sanding in between coats) If i drag a fingernail along the grain it still scratches. I coated it well over a week ago, so it has had plenty of time to dry. Is the scratching because I used such a dark stain? There are also areas that the varathane flaked off around the trim work. Did I do something wrong again? I have been refinishing furniture since I was a little girl helping my dad. I'm just not sure what's going on.