1) "Kol ha'Nashim" means that every woman in the world is "Dayan Sha'atan" -- they only make Tamei the food that they touched from now on, and not from before they knew about the bleeding. Because this Halachah apllies to every woman in the world, the plural form is used.

2) "Kol Ishah she'Yesh Lah Veses" does not refer to all the women in the world. It refers only to a special category of women, those who have a fixed biological time for their monthly cycle to start. Many women do not have a fixed time for this. This is why the singular form is used -- because it is a more individual question whether the Din in the Mishnah applies to any particular woman.

(As a parenthetical remark, I will note that I once heard that Rav Moshe Halberstam zt'l (who was possibly the leading expert in Yerushalayim on practical questions on Nidah) said that he had never heard of a woman in our time who had a fixed cycle for her entire adult life. Even though from the Gemara it appears that this was a reasonably common phenomenon, nowadays people are much less settled in their lives, and this also has an effect on body functions. According to this, I was thinking of saying, in the manner of Derush only, that we may understand why the Mishnah uses the singular terminology for the woman with the fixed cycle -- because it is quite unique!)

3) The Lashon Yachid used for the woman with a regular cycle does make a difference later on. This is below in the Mishnah on 11a. The Mishnah states, "Even though they said Dayah Sha'atah...." Rashi writes that this refers to the woman who has a regular cycle. How does Rashi know this? The Rashash there writes that Rashi was forced to say this by the singular phrase used by the Mishnah. Since the Lashon Yachid is used in the Mishnah 2a to refer to a woman who has a Veses, this seems to suggest that whenever it says Dayah Sha'atah in the singular form, it refers to the woman with the Veses.

4) This contrasts with the four types of women mentioned by Rebbi Eliezer in the Mishnah on 7a. There, the Mishnah states "Dayan Sha'atan" in the plural. We learn that the phrase "Dayah Sha'atah" fits with the woman who is not a member of this group of four but is a special case on her own; a woman who has a fixed cycle.

In contrast, "all the women" mentioned right at the beginning on 2a is not referring to any particular type of woman, but to all the women in the world, so the plural form is used.

5) My colleague, Rav Yechezkel Frankel shlit'a, explained slightly differently than I did, and said that "Kol ha'Nashim" does not mean all the women in the world, but rather all the different categories of women -- a woman with a Veses, a woman without a Veses, a Besulah, a pregnant woman, a nursing mother, an elderly woman (as listed in Mishnah 7a) and any other sort of woman. They all are Dayan Sha'atan. Since this includes all the categories, the plural form is used. In contrast, the woman with a Veses is only one category of woman, so the singular form is used.

Dovid Bloom