Pretty sure ZUN was just doing whatever came to mind during the PC98 games. The only works to obtain information from were the games, and the only semblance of continuity those had were reoccurring characters (Reimu, Marisa, Mima, Yuka, Alice, Genjii). In my opinion, it's safe to say that any attempt to apply logic to the PC98 games is pretty fruitless.
PMiSS states that the first use of the Spell Card rules was during EoSD, but this brings up another thing.
At around 1885AD, the Great Hakurei Border was built. Obviously, Reimu was not alive at this time. Sometime before 1998AD, the Vampire Incident occurred.
Originally, attacking humans was the point of the youkai existence, but since the erection of the Great Barrier it's become troublesome for them to do so, and they've had to find sources of food outside their normal choices; consequently, the collective willpower of the youkai began to drop.
It was into this situation that a powerful youkai from outside, a vampire, appeared, and quickly amassed a great number of subordinates.
These two events were more than 100 years apart. There must have been Hakurei maidens since the creation of the Great Border, and so if they were all human, it could be said that there have been at least three maidens, possibly including Reimu or not. In addition, the Vampire Incident was a good five years before EoSD, where the Spell Card rules were first used. Reimu did not know who Remilia was before the events of EoSD, and I'm fairly certain she had not previously fought any other vampires. However, the Hakurei maiden had defeated the vampire of the Vampire Incident to stop the youkai rebellion. This is the point at which the Spell Card rules were created, after the original contract was rejected by stronger youkai. It is doubtful that Reimu could have come to terms with so many youkai and have written such a deal. Mystic Square, circa 1998 (after the Vampire Incident), also did not use the Spell Card rules. So, if Reimu did not use the Spell Card rules that (presumably) she created, immediately after its creation, and no other incidents have occurred between MS and EoSD that Reimu did not resolve, who created the rules?
Reimu's mother, most likely.
