The operative words here are "...most people, sane ones..."

.
But I agree, there are a few options at play here, IMO.
1- There was no foul play in Tammy's death. Quick burial timeline was coincidental (along with all other suspicious circumstances).
2- Tammy's death was due to foul play, but Chad wasn't involved in or aware of it. It was carried out by Lori and/or Alex. Quick burial timeline was coincidental.
3- Chad was involved in foul play that caused Tammy's death, but he made a calculated decision about red flags raised by cremation. Without being part of the religion/culture, I don't think you can understand how unusual/suspicious cremation would appear, especially in a really traditional and devout family (Tammy's family, and by appearances, the immediate Chad Daybell family), and even more especially when you are making your living in the near death experience/resurrection/end times-obsessed fringes of Mormonism.
4- Chad was involved in foul play that caused Tammy's death, but hubris or visions (or both) caused him to believe he wouldn't be caught, so he didn't worry too much about evidence. Tammy's death was part of an important plan, and everything would work out fine.
MOO