But as a humble beginner, it is not on me to question the senpai, in particular not in class. But I can not stop thinking that there is something here that isn't entirely right.
Just a comment. While your attitude is widespread and encouraged - I think it's not a good one (to be clear: nothing wrong with what you do or said, I just find bad when "masters" encourage that "no questioning" approach
especially to beginners).
Imho questions should be encouraged. Sure, they must be asked without arrogance, and with the grace of knowing that in 99% of cases it's one's understanding that is lacking, not the method. But asked and encouraged nonetheless.
Every question is an excuse to detail a little, offer a single improvement and give the student a chance to practice that and get better. Probably at start 99% of the answer will pass over his head, but skill is built on the sum of the 1%s.
The idea of a school where questions are discouraged seems very bizarre to me, even as it seems widespread in the martial arts. The whole point of a master is exactly to have someone who avoids you to have to rediscover hot water every other week. If you don't use him, you can just as well watch youtube videos.
Unless I get an explanation from senpai, why this feeling is misguided(if it is), it's going to stay. At end of the day, no matter how much I respect our instructors, if it feels wrong in my body in terms of poor stability etc, then something can't possibly be right.
And this is absolutely the right attitude. If it's not working and you're shown that it's working for others.. you haven't yet been told the whole shebang. In fairness, often because people don't really know and they're just doing the missing bit without being aware of it.
Blessed is the student who has a master that can teach.
Also I have from physio visits, documented poor internal hip rotation, but at the same time I have outstanding external hip rotation (much better than most). So my overall rotation range is normal, the offset is just biased towards external. I think this is the explanation. This is why the standard angles just seems impossible for me.
For the same reason, I like the valeri style heel kick much more than the normal mawashi geri(I have to turn more than others to get the optimal angle). I have flexibility to pull this kick off as high as armpit level. Had I just been for flexible it would morph int on axe kick, but I lack flexibility in the other way to do that.
But OTOH, don't we ALL have our biases? So why not teaching constructing principles instead of angles from pictures?
Unless you have a specific health condition in your joints, there's no other reason for your hips to be more flexible in one direction than others, other that you don't train the other directions much, or you have habit that cause that stiffness (and that you can change). It's all muscles, tendons and ligaments and they all respond to proper training. There are anatomical difference but karate techniques cannot be based on having only a certain anatomy.
That said, it can take very long time for an adult to recover from their physical biases, and absolutely there's nothing wrong to stick to what works better for you
now. At the end, what we call "karate" was born with one goal: staying alive and come on top on in case of a unexpected, unwanted confrontation with one or more aggressors.
In my view, how many katas you know, how many stances you can take and if your feet are 45 or 35 degrees wins you no prizes.
I can experiment a bit with what you suggest, but when I feel unstable it is only when I focus on the look, more than the feel! Ie if push my interior hip rotation to it's limits, and my feets to the limit, to get "as close to the ideal angles as I can" in absurdum, then what happens is that I loose that feeling of sinking in deep, and I feel like I am pivoting on a pin in terms of balance. So this is the best I can do to get "as close as possible", but I FEEL that this is wrong, and in contradiction with what I THINK are the constructing principles of this stance.
If I OTOH, don't care about the exact angles, jsut lower my gravity and angle the feet by internal rotation of both hips and try to feel how my body pushes the "pyramid" into the ground, and I can "bounce a little bit up and down" and feel the tension, than I feel very stable, I use this stance often when holding mitts, to be able to better resist not foward strikes byut also angles attackes without moving stance alot. When I then look down on my feet, I am closer to sanchin of shotokan.
My hypothesis is that this is likely the "sanchin dachi" for ME that is most faithful to the presume constructing principles, even though it is not quite "as close to the looks" of the ideal angles.
Much of this is due to the fact that these positions were constructed for some practical and specific reason, but the knowledge of exactly
what was, at some point, lost in history; and the stances also mostly exist (as already mentioned by someone) only as fleeting moments in the constant movement and mayhem of a fight; they have little value or use in their static display, other than to train you to get in and out of them as quickly and efficiently as you can. But my impression is that such understanding is also often lost.
Both things allow for all kinds of odd, arbitrary details to be taken as a paragon of perfection. I'm a bit of an heretic perhas, but a lot of modern karate is, it seems to me, a bit of a Cargo Cult.
In other words, it's not the point that your (or the ideal) sanchin dachi works for you statically: it must work for you
in combat. That's really all that matters.