Agreed on all fronts. I think Seles's wins over Graf on clay in 1990 are much more significant.
I guess the big thing on where this topic winded up going to is the real question would be, not which of Graf or Seles is better on clay (obviously Steffi is greater with her RG wins, but who is better, particularly at their best, is much more in question, with varied opinions) but which would be a tougher opponent for Evert, and I think Graf and Seles are similar enough in their levels on clay regardless which you think is "better" it would be much more the match up and which is a tougher match up for Evert. For instance we know who is the far tougher opponent for Sanchez Vicario, including on clay (Seles, she even beat Sanchez more eaisly in their lone grass meeting than Graf ever did, lol), and we know who is the tougher opponent for say Novotna, even on clay (of course Graf, and even if Novotna played pre stabbing Seles on clay, like if she closed out Sabatini in the 91 French quarters she choked a big lead on, I see her doing better than she typically did against Graf, which was barely getting games when they meet on the surface), we know who is a tougher opponent for Coetzer (Seles of course, she was a thorn in Steffi's side when she was not at her best), for the Williams and Davenport in general (Graf of course, although both Graf and Seles should normally beat Davenport and Venus on clay where they aren't very good, even post stabbing Seles), for Hingis (Graf again), for Sabatini (Seles for sure).
On that I would go with Graf being tougher just because my general sense (again this could be challenged) is that Graf is a tough match up for Evert in that Evert lost their last 7 matches, winning only 1 set, which IMO not meant as a slam on Evert in anyway is not particularly good at all for an ATG, especialy a GOAT contender, against another GOAT contender. So my sense is just that Graf is a tough opponent for her period, we saw how much better an old Navratilova would do against Graf in comparision to Evert, which makes me get a complete opposite sense of Martina in the match up. While we will never know I would bet a lot of money if a time machine were invented to test this and every other theory people want solved in history out, even if you put a 33-35 year old Graf against a fully prime Evert, Graf would do better than this for sure, almost certainly would have a losing record of course, but would do better than a 31-onwards Evert did against a 16 year old or older Graf. I mean even Wade who was baby Evert's pigeon already, at age 32 beat her at Wimbledon and beat her a few times aged 30-32. So if both were in their primes together I would be stunned if Graf did not totally dominate on faster courts like grass and fast hard courts, and probably even carpet although a bit less sure there as Graf is more prone to some upset losses to much lower ranked players on carpet interestingly. Clay, slower to medium hard courts, and rebound ace would be more interesting, and of course clay is the only thing we are discussing now. By contrast I suspect Evert would not be the easiest match up for Seles, although here there is virtually nothing tangible to go on, just 2 meetings with BOTH players way outside their primes, which Evert crushed in one, and Seles won in the other. Nothing significant enough to draw much there. As I said earlier the main reason I think this is Evert is basically a much more powerful and more mentally tough Hingis, and while yes it was not peak Seles we do know Hingis was a very difficult opponent for her. So I feel comfortable in predicting Graf is probably a tougher match up for Evert, and Graf and Seles are close to relatively equal in terms of who is "better" on clay regardless which that is (I probably lean Seles at her best in fact) the tougher match up would be Evert's tougher opponent, which while purely I guess I would strongly suspect is Graf for all the reasons just stated. Now mind you this is where your pointing out Seles beating Evert on clay in 89 when Seles was 15, and Evert was 34, while 15 year old Graf was dominated in her matches by a much younger and arguably peak (despite that she was 30 in 85, Evert herself claims that was her all time peak in fact) Evert is noteworthy in this discussion and comparision. I just don't think on its own it is convincing enough if you were already leaning one direction as I was.
Of course both Graf and Seles would be considerable opponent for even the best version of Evert on clay, unless either were totally ill equipped to say the wood racquet era which is a possability and another unknown element, and both would challenge her and do some amount of damage to her regardless, regardless which did more. Of course the same is true in reverse though, Evert would likely do far better vs both Graf and Seles than any of their toughest opponents of their time- say Sabatini or Sanchez, and cut into both of their success on clay for sure.