I'm not sure that the US'91 SF affected Capriati that much; surely it was painful, but she won the Olympics against Graf the following year (her first-and only- victory over her). So I don't think she lost her motivation after '91.
The match that 're-started' Capriati's career was the =Z'01 quarters against Seles. She came back from a set and 2-4 to beat Seles for the first time in a Slam, and then won the other two matches pretty easily.
For Gaby, I would have to say the '93 RG quarters against Mary Joe Fernandez. she had it in her hands, and had beaten Arantxa, who was wainting in the SF, 6-1, 6-3 two weeks before. So she had a reasonable shot at least to the final, and after Seles' stabbing, people (and me too) were expecting her to step up.
For Hingis, it seems obvious that the RG'99 final hurt her a lot, but I think that the OZ'02 final was the one that really ended her career.