I uninstalled BtB's Game Settings, and found that I can no longer sleep outside since I get a 'Resting here is illegal' message. It seems that the GMST edits have stuck on my save.

Short of starting a new game, is there a way I can remove this from the save file?

You should be able to delete the changes in your save game using the Enchanted Editor.

Downloaded and installing, however I'm not sure what setting I need to remove.

Back up your most recent save game. Open the Construction set.
Scroll down to BTB's mod (it'll need to be in your Data Files folder). Click Details. Click the "ID" column top to sort it by ID name. Go down to the GMST section. These are all the GMSTs altered by the plugin.
One of them (or more) is your culprit. You can delete them from your save game using the Enchanted Editor in the Global Variables section. That should reset them to the normal values.

I'd keep a backup of your savegame though - if you accidentally delete the wrong thing it'll cause strange problems.

Alright, thanks!

Well I've found the culprit under game settings, however when I open up the save file with EE game settings is greyed out. What should I do?

Hmm, shout BTB's name into the abyss?

Seriously though, you've reached the end of my knowledge if EE won't let you edit that part. You might want to PM BTB or ask on his release thread.

If you change the GMST, it should revert to normal after the cell resets, I think. I'm pretty sure I did this a few months ago.

That might work. Do you know of a way I can change the respawn time to less than 3 days?

Just rest for three days to see if it works. It only takes a few minutes real time.

Ugh, I've tried in both Fargoth's house and Todd Test, still can't rest in the Bitter Coast.

I give up, I'll just have to find a mod that makes Arielle rent Beds.