Worriedandlost,
I was 17 when my technique got totally picked apart and reconstructed in four lessons and lots of practise (I had to fly to England for those lessons). The teacher I had then was top-class, but he made a very valid point to me: that everyone's body is different, that this was a way of playing but he could only demonstrate the concept and the sound and expected me to put the work in and figure out how to reproduce it for myself. He explained things like how changing the contact point, speed, pressure and angle of the bow changes the tone you produce, for example, but expected me to go practise until I could consistently get the particular sound I wanted. I learnt that there are different sorts of vibrato, that the width and speed can be controlled and that doing this changes the sound you produce. And so on. But it was always "this is a way of doing things; it works for me."
I'm now in my 30s. I probably don't play as well now as I did then, technically, but if I wanted to, applied myself and really practised, I could be a lot better even without an external teacher - because I understand more completely what I am trying to do and the physics involved.
The chin thing - I had shockingly bad technique for a while, and used to squeeze the violin and bow with a lot of tension. It caused me real pain at one point and I still have vestiges of those problems. Most children know that they don't need to squeeze, but I was paranoid about dropping the instrument, I guess. Because of that, learning to play without my chin on the violin at all has been really liberating, reduced my tension all-round and means that now even when I do play chin-on (most later music, which requires shifting/vibrato), I don't do the nasty squeezing thing that I did in my early teens.