a trip to visit sokushinbutsu

Hi everyone,
I'm planning a trip to Japan, part of which focuses on the mummified buddhas known as sokushinbutsu. (I'm a writer; this is a research trip). My plan would be to use Tsuruoka as a base, rent a car, and visit a few of the most special temples. I wondered if anyone here might have any tips.

a) As of now, my list includes Dainichibou, Churenji, Nangaku-ji, Honmyoji, Kaiko. Are there other temples that people might suggest? (Even if they are outside of Yamagata)

b) As far as timing goes, do people know if there would be seasonal restrictions to a visit? My plan was to make the trip somewhere between mid-April and early May. I've been reading that access to Dewa Sanzan is very much seasonally dependent, but wondered if that applied to the sokushinbutsu temples.

c) Because my interest in sokushinbutsu is closely connected to a larger interest in shugendo, are there other places I should visit in that area that are particularly evocative of shugendo? (I know Dewa Sanzan is generally connected to shugendo, but I'm curious about specific sites).

OK, I think that's it. Thank you so much!