I have amended the text of the FEP to clarify a couple of things, but also changed the inheritance logic following this month's WG meeting and subsequent discussion on ActivityPub.space.
Instead of recommending that replies inherit context from the object it is in reply to, implementors must find the root node (how, is out of scope; tree traversal or context resolution are two ways that come to mind) and inherit its context.
This will simplify context resolution and pave the way for other actions like moving, crossposting, forking, locking, etc.
I also added in a blurb about situations in which a context would explicitly not be inherited.