It is my understanding that the editors do their part when at least a rough draft is finished.
Not in the case of Trek fiction. The editor is involved from the start of the process, either selecting writers to do projects the editor has in mind or considering pitches from writers and deciding whether or not to approve them. The editor may then suggest revisions to the outline before the manuscript is written.
And as stated above, in the case of an ongoing series like the DS9 novels, the editor's role is somewhat like that of a TV showrunner, shaping the overall flow of the storyline and guiding and coordinating the work of the various individual authors. And yes, that does mean that authors are often given certain key story points they have to include, though it's up to them to figure out how
to do it.