I'm not an expert on medicin and diseases so I can't come up with anything which contradicts what you are writing.The food should be out after a few days, it takes months to get scurvy due to vitamin c deficiency.
The worst that can happen due to a high intake of vitamin c is is some diarrhea and of course if he gave them almost toxic levels that means non-toxic so it's not an issue.
Replicating medication would obviously take priority over other things, especially if it affects the entire crew. There's no believable scenario where they wouldn't just do it.
No, it's awful. The writer should have come up with a made up illness or at least have picked one that isn't super easy to treat.
But you should read the book. It's very good.