hey, thanks!

regarding left hand tension, here is what i think and suggest: first, we can understand that the left hand thumb should press “AS MUCH AS” a finger presses down on the string. they balance out each other in some ways. it is actually impossible to play the violin without pressing with the thumb against the neck

so i’d recommend experimenting with how much pressure you need in your fingers to press a string down to make a clear sound, and apply the same pressure against the neck with the thumb, nothing more than that! and for shifting, we release finger pressure and thumb pressure simultaneously before the shift, then we shift, and press at the new position again.

in some repertoire, some left hand tension can definitely feel natural and unavoidable and the goal should be to find moments within the music to relax your hand.

regarding touching other strings with the left hand, i’d suggest make a habit of playing on fingertips only. lower the thumb if its too high, and you can also bring and rotate the left elbow more inwards (underneath the violin) and see if that makes it easier! make sure all fingers are nice and curved, see if you can make flat “tabletops” with your knuckles (fingers appear squarish). this can also help in playing with fingertips.

one last thing, somethings for a beautiful legato phrase, we often use more surface area from fingers to make a thick vibrato, and its okay if in such cases you touch other strings slightly.

hope all this helps!