Does a sensei need to strictly insist his style to his students although a lot of variations are open for everyone to choose from where they think is satisfying for them?
I will only speak to my main base, Doshinkan aikido (Yoshinkan). The form is the form. One of my favorite instructors taught last night, and what he said struck me strongly..."be as relaxed as possible while still maintaining the form". So I see a lot of what I'm working on now as trying to be more and more relaxed while having better and better form. That creates an interesting tension in my body, my mind and my training. Right now, that tension is very important for me to work with, and if I abandon the form I lose that.
In another context, using a variation might very well be most appropriate. But in general, I don't want to do a variation while in training. I want to do the waza as shown...to learn from someone who has gone before
. If all I want to do is my own thing, I can hang up a shingle and see who's willing to come and sample what I do.
Doesn't the student have the right to choose what variation they should use but to accept what the sensei strictly insists (on such style patterned to what he had learned 2-3 decades ago and then refused to learn further)?
I'm surprised to see that you think an instructor has stopped learning for 2 to 3 decades. I don't think I would stay with such an instructor. The Yoshinkan focuses very strongly on form...but personally, I think even with that stricture, the serious teachers and students keep pushing the boundaries of that form, and the relaxation it contains. I don't know your situation...but if this is not happening, you may want to either
a) look deeper into the situation, or
b) find someone still developing themselves so that they can lead you.
It could be that you are missing what is going on...I couldn't say from this distance.