Have you talked with your sensei or senior students about what is expected on the test? In my opinion the issues you seem to be concerned about are completely inappropriate for a gokyu test.
I know when we test people at that level we are looking for an understanding of what is being asked (ie you know the difference between tenkan/irimi and kotegeashi/nikyo etc), a decent knowledge of the appropriate forms, and a base understanding of the principles as followed by our organization.
Is it considered acceptable for the nage modify the technique in order to unbalance the Uke during a test?
Ok, to actually provide an answer to your question I would say for a gokyu level test, absolutely not because the uke on a gokyu test shouldn't be providing resistance IMO so you shouldn't have to worry about overcoming it.
For a higher level test, absolutely yes