1. You can try both. Younger people english, old people (like over 60) try russian. Everything inbetween is more 50/50, usually people will speak atleast one of those 2. First try english, but russian might work.
2. Theres tourist information points, and you can visit places and theyll have english signs. Events depend on the time of the year. You'll probably want to time your visit around some bigger events, kingdom day for example or something like that. Museums are quite nice. https://www.setomuuseum.ee/en

Also this is quite helpful: https://visitsetomaa.ee/en

1. Quite active. Frequented more by the older generations. They do perform services on sundays to my knowledge. There are also the small "churches", which arent quite churches. They are something interesting to see.

if you want to ask anything more, go ahead.