I was wondering if anyone knew or could direct me to a guide on how to
correctly update/edit the global and character Lua files?

I have a list of quests that I know that I've done, and instead of
ticking them off the list in-game (as there are a lot) I am hoping
there is a way to add them to 'completed' lua files found in the
account and character saved variable folders.

I have tried adding just the QIDs, I have tried adding the quest names
in the same format like:

["Riverpaw Gnoll Bounty"] = true,
["Further Concerns"] = true,
["Find the Lost Guards"] = true,

But no luck - it seems I just can't find the right combination - or
maybe it's just not possible.

Any help would be greatly appreciated! :)

Adding the QIDs should be enough, but I'm not sure if that will work or not.
 If you know the quests you've done, it's probably simplest to just load the
guide and skim over it checking off everything you've done.  Faster than
looking up QIDS or exact names, that's for sure.