New post from the devs: https://playnightingale.com/news/accessing-legacy-mode
Updating my post to follow the official instructions with a little Discord flair: https://inflexiongames.zendesk.com/hc/en-us/articles/26890830706203-How-to-Copy-your-Online-Mode-Character-to-Offline-Mode
These steps will walk you though logging into Nightingale's account linking page, saving your online character files and moving those files into your local offline save folder.
1️⃣ Navigate to https://auth.playnightingale.com/auth/linking/ and login.
2️⃣ Scroll down and click on your character name.
3️⃣ Save the zip file then locate it.
4️⃣ Right click on the zip file and select "Extract All..."
5️⃣ Click "Browse"
6️⃣ Paste %localappdata%\nwx\Saved in the folder address bar.
7️⃣ Press 'Enter'
8️⃣ Click on "Select Folder"
9️⃣ Click "Extract"
You can now load up your character in offline mode! 
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