Great moments in Torah Commentary

At the end of last weeks parsha, [Numbers 15:39] Rashi tells us why we wear tzitzit:

The total numerical value of the word tzitzith is six hundred, and together with the eight threads and five knots you have six hundred and thirteen [correspondiong to taryag, the six hundred and thirteen commandments.]

Pretty straightforward, right? Only the Ramban has a serious objection:

I do not understand this, for the word tzizith in the Torah is written without a [second] yud so the total numerical value is only five hundred and ninety! Moreover, the total number of threads in the opinion of Bes Hilel, is only three and the knots per Torah law are only two.

The Ramban's objection is facinating as it raises the following questions and implications:

1 - Did Rashi have a text of the Torah with a different spelling for the word tzizith? Or is he just playing a little loose with the rules of gematriah?

2 - Of course, Rashi knew that tzizis weren't required by Torah law to have eight threads and five knots; he knew that this arrangment was created by the Rabbis. (After all, he studied Menochoth.) Yet, on this verse Rashi suggests that tzizis serve to remind us of the commandments, because they have eight threads and five knots! Well, what if the Rabbi's had created another arrangment? The math would have turned out differently! And if the math had turned out differently, how, per Rashi, would tzitzis remind us of taryag? Is Rashi suggesting that the rulings of our Rabbis were inevitable, and not contingent? Doesn't this view obviate free will?