Marshmallow firmware should work fine, though nougat firmware is recommended
RUU route is recommended. But there is a ruu thread and you could always check the way to get it via twrp or fastboot from there.
Since you are S-On, you can't. It's one of the perks of being s-off.
Anyway, viper seems to be no longer maintained.
And to be fair most sense based rom on HTC seems to be reaching its final release since there would be no official updates anymore from HTC. Only aosp based roms would have better chance to get the latest update. But even that isn't guaranteed.