A TV movie wouldn’t have to waste any time setting up the major mysteries and then resolving them, since the end of Season 3 already did the heavy lifting of setting up.
Honestly, any smaller mysteries could be resolved with Jeff Rake tweeting them out, if that would mean a solid movie!
The fact that Rake has been planning for six seasons all along means that the show has been building toward an actual ending.
Any mystery minor enough that it wasn’t included in the Season 3 finale could probably be cut out of the story, folded into the larger plots, or just mentioned in passing to tie off the loose end.