Yoma 8a - Haza'ah all seven days

The gemara says that if you hold טומאה דחויה בציבור and that טבילה בזמנה מצוה, you'll need to do haza'ah for tumas meis on all the days of hafrasha, in case they are the third or seventh days. Rashi explains that since tumah is dechuyah, בעינן טהרה מעלייתא - we need the best possible taharah.
That seems remarkable to me. Is it really true that the taharah is greater if you do haza'ah on the correct day? I grant that it's a big mitzvah, but how does that upgrade the person's taharah? Isn't he either tahor, or not?
לשבר את האוזן, it's presumably a big mitzvah to give tzedaka before the haza'ah, but would anyone imagine that it is therefore required because of טומאה דחויה בציבור?

A separate question: Rashi says
Now I didn't think that was true: I thought that R' Yochanon says (daf ד) that you learn hazaah from miluim, and Reish Lakish says that there is no source, it's just מעלה בעלמא. Nobody holds both.

Then the gemara says that if you hold like R' Yosi and if he holds that it's הותרה בציבור, he shouldn't need haza'ah at all. Well, why not? After all, he learns from miluim. Is it somehow true that if there's no point to it, we know that we wouldn't learn it out? And didn't Rashi just say that holds הותרה and nevertheless requires haza'ah?

I'm just not following the steps in the gemara well.