You just touched on the major problem with teaching mental intent. Everyone's thoughts are just so subjective that it is extremely hard to get the exact same result between two people using the same words or mental image since, as you say, each interprets it differently. However, your suggestion to break it down into smaller stages that include feedback from an external source for correctness of feel, is a very logical way to approach the problem and I have had some success with that method recently.
I'm not even sure it is really about teaching - it seems of utmost importance to us as students. We get to see and work with world class teachers very infrequently. So maybe when we do, okay, they put us through our paces for a day, we feel like we saw and felt some awesome stuff, but a week later, it's like, what did I actually learn?
If anything seemed vague or I am dependent on traveling to someone or them traveling to me to get correction and ensure I'm even training the right thing in the first place, it seems like a losing battle that can't be won. At least if it is broken down to where I think I logically understood what someone was trying to teach me, I can measure improvement and diagnose my own problems way easier in the in-between times - because the person took the consideration to teach me how to teach myself.
But if it's just - here, do a bunch of these and then come see me in a year - the probability of me screwing something up badly will be nigh on 100%. Wasted effort for me, wasted effort for the teacher.