What is istihatha?
The discharge that usually comes after the hayth, but it is no longer defined as the hayth.
Is there any evidence of the scholars’ limitations of the minimums and maximums of menses?
No, there is no evidence. Ibn Taymiyyah pointed this out. If there is no evidence for limits to cycle lengths, why put a limit?
What is nifaas?
It is the blood that is discharged after childbirth.