Lots of good questions.
I always layered a sealant over a glaze when I used them and it was for the sole purpose of hiding defects.
The first I used was 3M Imperial Hand Glaze. I don't remember much about it other than being pretty easy to apply by hand and when under a carnauba wax (100 Grand Blitz to show how long ago this was) it looked pretty good. No idea on how durability was effected since the wax only lasted a couple of months anyway. From there I used Poorboys World Black Hole and Prima Amigo. The Poorboys had better filling, but Amigo had a bit of correction, so it would clean up the paint a bit better. Both were designed to be topped, so they didn't have any effect on the durability of the sealants I put over them.