2- I think that I setup DevonThink so that children inherit the parent’s tag and that notes automatically inherit the group name as a tag. I may have done this in Evernote many years go before importing in DevonThink. I can’t remember. Would you know if I can configure tags in the DevonThink settings ?
Yes, you are using group tags because you have File > Database Properties > Exclude Groups From Tagging disabled and potentially enabled Inherit Tags of Groups.
3- I have a zillion tags. I imagine that if I delete many, there is no risk of deleting the notes containing those tags.
Correct.
This is all discussed in the Getting Started > Tagging section of the built-in Help and manual