You have made some good observations regarding the position of the palm (or, relationship of the hand to the forearm) and how subtle positions can augment or embellish service effects.
I have long stated that the majority of pros simply adjust this aspect of their hand position as opposed to making specific grip adjustments. (There are examples of both subtle grip changes as well as turning the hand in or out more.) Players should experiment with different position of the palm (curled in more for spin or turned out for flatter serves as you have correctly reported), as this is where players can discover aspects of the serve that contribute to more effective serving abilities.
Thanks for bringing this important point up. It is a good one.
P.S. With all the holidays, I've been doing the 'family things' of late. Hope to visit more often!
Author, Tennis Mastery & Coaching Mastery