But again, IMO, that's all just about compiling numbers. And, sure, if we're going just by numbers, Graf wins.
You also note that we can't focus on "what if's." But, as I've said before, the absence of "what if's" works both ways. Sticking to what actually happened, 16-18 year old Seles won the French Open from 1990-1992, beating Graf in the finals in 1990 & 1992, right after Graf had won 8 of the last 9 Majors.
I would assert, and many would agree, that Seles had surpassed Graf on clay at that point in time.
The unanswered question is whether Graf could have taken back her clay throne had Seles not been stabbed. And the only way to answer that question in the affirmative is to use a "what if" and speculate that Steffi could have done this. We have no concrete evidence that Graf could have surpassed Seles on clay had she not been stabbed.
What we do have is Graf reclaiming her clay crown right after the stabbing by eking out a win against Fernández in the French final in 1993. If we're focused just on numbers, Graf and Seles were tied on clay at this point b/c they each had 3 French Open titles. But I would say that tight win against Fernández actually weakens Steffi's clay case.
Well first off I don't do what ifs at all, I am not like you, insideoutforehanders, and others on this site who seem obsessed with them. I believe you the one making a bunch of posts what would have happened with World War 11 era players without the war and insideout seems to start a new one each week atleast, so people like you and others here are obsessed with that kind of stuff, which is fine, but that isn't my thing. OK as fun speculation on things I will, but when ranking players hell no. So that is the first disclaimer. You won't find me doing that even for a player I like, so I am consistent.
Now sticking to what actually happened, to me it is simple. Graf has 6 French Opens, Seles has only 3, a huge difference. Graf is ahead in every single clay stat. Except for head to head, where Seles is still not ahead, merely tied. I also find the argument that Graf taking 3 sets to beat Fernandez in a RG final as weakening her case is utterly laughable. Seles had 6 different 3 setters in her 3 French Open titles, including ones with such giants as Kijimuta (the weaker sister of the Kijimuta sisters too) and Kelesi, far weaker players than Fernandez; in addition to a 2 tiebreaks match vs Meskhi. Fernandez is also the same player who lost to Seles after having a match point and making her eat a tasty bagel during her so called dominant run at the Australian Open, and who Seles barely beat 7-5 in the 3rd set, in their only RG meeting in 97, and only since Fernandez choked at the end, making a ton of errors from 5-4 up. Overall both totally dominated Fernandez in their careers, but Graf is even more dominant against her if anything. So yeah is one of the crux of your argument for Seles > Graf on clay all I have to say is LOL. And if your argument is peak Seles is superior on clay, I think that is dubious at best, and I wouldn't even agree. Graf at the 88 French for instance had a far more dominant performance through the draw than Seles did at any French Open, it isn't even particularly close in fact. Her 96 French Open title was also overall more commanding than any of Seles's 3, even with the near loss in the final, considering the total ease of the rest of the draw. So would disagree even then, but even if I did it would not overcome Graf's giant stat edge.
PS- Bud Collins who was a total Seles worshipper to pathetic degrees, and known Graf hater, and also a tennis icon whose extensive knowledge on the sport easily dwarves both of ours, all bias aside, even ranked Graf 3rd in a best clay courters of all time list he made behind Lenglen and Evert, and Seles 7th, so take that what you will.