I absolutely think that your gentle approach to her head shyness is the correct one. When she relaxes and lets you touch her ears or poll, don't insist and keep touching but rather touch her briefly and then give her the reward for being good, which is removing your hand and giving her release. That is using positive reinforcement to teach her, which is always a strong motivation for anyone, not just a horse lol. The seller's approach was to punish her for being head shy, which in my eyes will only make her more tense about having her head touched as quite probably she wouldn't associate the punishment (tugging on her head) with the perceived offence, and just learns that people round her head are unpredictable and scary.
I'm sure she'll learn quickly. It's natural for a horse to dislike having its head messed with, and if no-one has bothered to teach her otherwise then she's never got over this reaction. But once you spend a bit of time on this daily and she realises that you're not a threat and you're not going to punish her for jerking away, she'll quickly learn to relax with you. Absolutely agree also with Sky's comment that if she does jerk away then you shouldn't step back also, because that would be a reward in her eyes. You should stay close but give her a moment to calm down and then try again.
There is nothing more peaceful than watching a horse eat.