What does a user see when a before-submit user event script throws an error, and is their form data kept? When a before-submit user event script throws an error on a save from the NetSuite UI, what does the user actually experience? From what I understand, the user lands on NetSuite's error page with the error message, and nothing is saved.

Your assumption or understanding is correct. • the user lands on NetSuite's error page with the error message, and nothing is saved.

So now i would need a client script to keep the UI running and a UE script before submit for CSV /APi uploads edits?

That is the right approach if you want the user not to lose form data yes.