Regarding Henin and Hingis, Hingis won 3 out of her 5 slams in 1997 which was one of the weakest years in the history of women's tennis. Her competition was pretty thin on the ground that year. Henin won her 7 slams between 2003-2007, and all 5 of those years had a stronger level of competition than 1997 did.
Also Hingis won Wimbledon and Henin didn't, but Hingis got her title in 1997 before Davenport, the Williams sisters, Mauresmo etc peaked. Fair play to her for that though. She beat an injured Novotna in the final that year. I really don't think Henin was any worse a player on grass than Hingis was to be honest. At her peak she had to contend with far stronger grass court opponents than Hingis did during hers.
The two players were of a pretty similar standard on hard courts, grass and indoors but Henin was clearly much better on clay.