Re: bad technique vs. resistance
This is how we are taught.
In training you should grip tight enough so that nage struggles to do the technique, then you should let off a bit so they can do it. Over time the technique improves bit by bit and the grip can get stronger and stronger. This is the Iwama Ryu way of doing things.
Uke gets stronger, nage's technique improves.
You did the right thing to not deviate to a different technique. The uke should, in my opinion, let off the tension a bit and let you do the technique to the level you are capable of. This is how you improve.
In future, maybe ask them to hold a little easier? So you can actually do the technique.