*hratan
ā becomes o in Middle English, and combined with the evidence of ratten I'm more inclined to reconstruct a short vowel and a long consonant for Old English, going back to Proto-Germanic *(h)ratt-. Where this comes from or what it's related to is anyone's guess, but it can't be related to *hrītaną on account of the ablaut.
Look, you are falling down the rabbit-hole of regional variation. There is a form rit out there and a form rait, and rot, and rawt, and ret. The form rit is very common in Northern England (i.e., Scotland) while the form rat is more common in the rest of England and that's why I entered it as rat. There are a lot of forms out there for this word, but all of them seem to derive from *hrītaną. You are right, I should have just entered the Old English form as̽hrītan, because this is what it probably looked like when you discount all regional variation.
By the way, I originally listed the Old English form as '̽hritan' but then I went down the rabbit hole of regional variation where me thought that a form ̽hratan might have existed, as well.
On second thought, it is also unclear where the verb rit comes from. The Middle English Dictionary says that it derives from unattested Old English *rittan not *hrītan. rit is pronounced with a short i, by the way, and rat also has a short vowel.
Another thing that keeps bugging me is that the verb rat has a form to-rat which is very similar to Dutch terijten and German zerreißen and as far as I know the verb rit does not have such a form. Why would it have such a form (i.e., to-rat) if it did not come from *hrītaną? I mean, there is a rather small number of words out there that are prefixed with to- in English (including dialects and late Middle English) and almost all such words have cognates in German, Dutch and other Germanic languages. How would you explain that?