First of which, we can tell that it isn't Suzuha because she can't recall ever going back to the 2000 and wouldn't even need anything from then, we all speculated that it was a different Suzuha going back to find the IBM 5100 but if that was the case then she would have gone to that time in any worldline, so I believe the first John Tutor was none other then our favorite mad scientist, Hououin Kyouma! But why?
1. Only he would know what would happen in the future, survive and be able to make himself believe what he is saying, we don't get all the things he said but some of them could have appeared on the news.
2. Okabe would know that he himself would soon meet Suzuha, Okabe makes the assumption Titor would know him.
But then if this is true which Okabe is John Titor?
Can't be the Steins;Gate one, he has a happy end
Can't be the Beta one, he no longer needs to do that
Then what about an Okabe that lost Mayuri instead of Kurisu?
He would stay in that EXACT worldline and know every thing that would happen!
Then how did Suzuha get that username?
If you go back in time you also go back in a past world line. Different theory for a different time.
Anyway, EVERY OKABE PLANNED THIS END. Well almost all of them. The one that got a happy end was just the lab rat.
Experimenting on himself, Okabe truly is a MAD SCIENTIST!