This is all relative. Chris Evert reached like what 10 Wimbledon finals? People think grass is her "weakness" but that's only because she had Navratilova as her rival all those years.
Seles already beat Graf in two French Open finals at age of 18. What if she wasn't stabbed, how many French Opens would Graf have actually won?
Either way Graf is clearly superior to Navratilova on clay and Evert on grass. Nobody would even argue this. On the other hand Graf is also clearly inferior to Navratilova and Evert on their "best surfaces" and arguably inferior on the majority of them.
In any era there would be someone better than Evert on grass. Court and King would have beaten her the majority of time on grass. King didnt have her first long to Evert on grass until she was 33. Even Goolagong owned Evert on grass in her prime, only losing once (8-6 in the 3rd) before 1978. Graf would have regularly beaten Evert at Wimbledon, then later Venus and Serena as well. Connolly would overpower her on grass, while Hart and DuPont probably would be atleast her equal on it. Lenglen and Wills would also be superior. There isnt any era she would win a huge number of Wimbledon titles. If anything she was lucky to have her best years where she won 3 Wimbledons in the era where the inconsistent and non dominant Goolagong was her main rival for the most of it, and not during Navratilova's prime or any of these others primes.
As for your latter question I would say something from 3 to 5, but more likely 3 or 4 than 5. She had 2 when Seles was stabbed, Seles wasnt going to win 10 French Opens in a row, so someone else would have had to win a few that decade and Graf would be by far the most likely. Seles would easily have more French Open titles than Graf though I think.
I did bring up the latter point on the best clay courters thread to why I ranked Graf lower but you didnt seem to agree with it, so I am surprised you are bringing it up now. Given that you ranked Graf #3 on clay behind only Evert and Seles, which was higher than I would have, you are already confirming even more Martina is nowhere near as she wouldnt even be top 10. I would rank Graf #5 all time on clay behind Evert, Henin, Seles, and Lenglen, but the greater consensus (which I disagree with but it is what most believe) is that she is #2 all time on clay behind only Evert. Navratilova would definitely be behind all of Graf, Seles, Evert, Wills, Lenglen, Court, Connolly, Henin, Sanchez Vicario, and several others on clay.