Some quick thoughts …
Druids does not have any historical connection to Bohemia, although old Slavs had similar institution of “wise man” called “Žrec” the christianization of Bohemia was a completed process for some time.
Same goes for strictly pure-pagan people. I would say that with highly rare exceptions EVERYBODY was a christian. But that obviously does not mean there is not a room for heresy or sects, but I would say that most of them would have roots in some christian dogma. Also it seems that there was a great deal of mixing the old with the new thus performing traditional rituals with christian artifacts (such as waffer, holy icons etc).
Inquisiton is a big topic, but the that famous spanish inquisition didnt exist in 1403. There was papal and bishopic inqusition but that wasnt an institution or organization it was more a way to perform investigation. And it was quite revolutionary at that time, removing trials by ordeal and instituting interrogations of wittneses and suspects, collecting evidence and a form of tribunal to carry out a verdict. Yes the system was not perfect having the investigator, prosecuter and judge in one person was not exactly a good system solution.
Herbalists or apothecary are very plausible game characters due to the fact there will be some kind of alchemy. But dont expect small hut in a swamp with human bones scattered around and old, crooked witches dotted with warts 