How did Chazal know that Dinah was missing?

(32,23) "…and he took his two wives and his two maidservants and his eleven children." Rashi asked here: But where was Dinah? He put her in a chest and locked her in, so that Eisav should not set eyes on her.

The question is asked how Chazal knew that the one who was missing was Dinah - maybe Dinah was one of the eleven children, and one of her brothers was missing?

But behold, the gemara writes that the reason why the Beis Hamikdash was built in the territory of Binyamin was because he had not bowed down to Eisav, since he had not yet been born at that time. Now, if one of the brothers had been missing, then he too had not bowed down to Eisav, and if so, why was the Beis Hamikdash built specifically in the territory of Binyamin, and not in the territory of that missing brother? Perforce, we have to say that all the tribes were there with Yaakov, and they all bowed down to Eisav, except for Binyamin who had not yet been born. And therefore only Dinah was missing, who in any case did not have a portion in the land. Thus, the question "where was Dinah" is justified.

Was the fifth son of Seir called Dishon or Dishan?

(36,20) "These are the sons of Seir…Lotan and Shoval and Tzivon and Anah. And Dishon and Eitzer and Dishan."

Behold, from here it seems that the fifth son of Seir was called Dishon (with the vowel called a cholam), and the seventh son was called Dishan (with the vowel called a kometz). And this is what it says also in posuk 30. However, when the Torah enumerates the grandchildren of Seir, it says in posuk 26 "and these are the children of Dishan", apparently referring to the fifth son of Seir according to the order of enumeration, but in posuk 28 it says also "these are the sons of Dishan", apparently referring to the seventh son. It calls both of them by the same name, Dishan, and so it apparently mentions the sons of Dishan twice, but it does not mention the sons of Dishon, the fifth son, at all!

However, we can explain that Dishon in posuk 21, and Dishan in posuk 26, are one and the same. But since in posuk 26 the word has a note called an 'asnachta', it changes the vowel to a kometz, in the same way the word for heaven - shomayim, changes to shomoyim, and the word for water - mayim, changes to moyim. That is, at the end of a posuk, and in the middle of a posuk when there is an asnachta, it changes to a kometz.

The proof for this is in Divrei Hayomim, because there when it enumerates the genealogy of the children of Seir, it writes in posuk 41 and posuk 42 that both the fifth son and the seventh son were called Dishon, because there neither word has an asnachta.

What emerges from all this, is that both the fifth and seventh sons of Seir had the same name, and they could variously be called either Dishon or Dishan. But the rule is that in the middle of a posuk it is vowelised Dishon, and at the end of a posuk or with an asnachta, it is vowelised Dishan.

This rule is sufficient for the sons of Seir. But the son of Anah, who is the grandson of Seir, was always called Dishon. And therefore, in posuk 25 it is written Dishon even though it has an asnachta. So too, in Divrei Hayomim in posuk 41 it is also written Dishon even though it has an asnachta, because the son of Anah was always called Dishon.