Re: how many back talk would you take?
I'd suggest a private, straight talk conversation with the guy. Tell him Sensei isn't satisfied with his shihonage because it doesn't fit with the way you do it in your style and that it's not really about what you or he think is best, it's about whether he's doing aikido according to your style. Tell him Sensei has asked you to work with him on this and make sure he's got it down before the test.
Then ask him if he's willing to learn this from you. Then shut up and let the awkward silence go on as long as it has to. Don't rescue him.
If he says yes, you have your agreement. If he can't give you a clear "yes", tell him and your Sensei that regretfully, you can't do it.