I tend to find that the best way to make sure I am doing the correct Kata is for Uke to attack strongly enough to hurt me with their strikes or put on full resistance when attacking with a grab. Without a strong, committed and resistant attack, I can do things that are not good Kata and get away with it. When the attack is serious, I have to do good Kata or the technique doesn't work.
For me, the Kata is the demonstration of the principles and the principles are there to ensure that the Kata works. If a novice is trying a technique and you allow the novice to complete the technique even though they are not doing the correct Kata, you may keep them from being frustrated but you are also helping them learn bad Kata. If the novice is getting frustrated, it is not up to the Uke to lighten up but for the Sensei to deal with the problematic Kata. As long as Uke is not trying to stop Nage by switching the attack or their balance all the time, the novice Nage should be able to learn the correct Kata.
On the other hand, that may be why we have so few students in our Dojo and lose so many after a couple weeks/months.