Vanessa, although I agree with you that it is too presumptuous to assume that there is an Ex-Warlord out there, it's also totally plausible for there to have been one and then lost it within 5 years. Even if you don't believe that, nobody said your theory about when the Shichibukai were formed is true. In fact, your theory says that Ace was offered to become the 7th warlord, and that there were only 6 before that. This was only a few years back, why did they randomly decide to upgrade in number? If the number of pirates in the Schichibukai can be changed like that then why did the government freak out about there being "a hole" or "a missing spot"? Therefore, if you believe that the # of warlords is significant enough for them to just not add someone for lulz, and you also believe that there was no 7th warlord before Ace's invitation, the most plausable conclusion is that the warlords weren't even finished being made when Ace was invited. Because of your first theory about when the warlords were established(which I actually agree with), your second theory about there being no warlord before Ace is at the very least questionable.
But, when the theories all come out, and when our thoughts are all spilled into a thread, there is only one absolute truth, nobody fucking knows.