Sorry, but the 'keeping cards close to chest' argument holds no water with me. If he was doing that, why bother to lie about the Minbari at all? Why not simply not point out ANY race as being the oldest? The way you phrase it seems to imply that he had to either lie and say what he did about the Minbari or else reveal spoilers about the Vorlons, ignoring the possibility of just keeping quiet altogether. There would be NO reason in the slightest to say the Minbari are the oldest to cover up thain fact the Vorlons are the oldest, and quite apart from the fact that I personally doubt JMS ever directly lied to the fans, the only logical conclusion as I see it is that at the point he made the statement, it was true. The early B5 brochure linked somewhere above certainly indicates that the Vorlons went through changes during the development of the show.
You're confusing the two arguments. I never said that when G'kar says in the pilot that the Minbari were the oldest federation, that JMS was lying. At that point, as far as G'Kar knew, the Minbari were the oldest because in terms of the storyline at that point nobody knew a lot about the Vorlons, except for the Minbari. When I said JMS had been "keeping his cards close to his chest", I wasn't inferring that he was lying as you seem to think I am. G'Kar, as far as he knew, was saying the truth. JMS was doing a slow reveal on the Vorlons, which culminates in later seasons as we discover things about them at the same the characters do. Remember In The Shadow of Z'Ha'Dum? Would that have been as big a revelation if we had already been told that the Vorlons were billions of years old? Of course not. We may have suspected from clues that JMS gave us, but we didn't know for sure.
The promotional brochure, which I quoted, was for the WB execs, not the fans, so he was not "lying to the fans" with that brochure. Its already been stated in the official magazine that, when he was trying to sell the show, he was worried about how TV execs would take to the ideological aspects of the show. And in the pilot and the during the first season, not a lot is known about the Vorlons, so he couldn't put on the brochure "nobody knows anything about the Vorlons" and leave it at that because that's not going to help to sell the show. So the stuff about Vorlons conspiring against Earth in. Maybe they were older ideas, but I find it so hard to believe that twelve months before filiming on The Gathering began that the concept of the Vorlons, and hence the whole of the B5 story, was so different. Sorry, I just don't buy it.
Of coure, when JMS releases his scripts, with the original Gathering script contained inside it, maybe we'll be able to find out for sure. But its fun to speculate, and its fascinating to see how B5 was developed.