You're obsessing over "teaching" her stuff she already knows how to do. She knows how to pick up her feet, she's not a young foal or an unhandled horse. You are just nagging her. If you need to pick up her feet to clean them, do it.
This, with everything, the mare knows this stuff, and what you are going to do is end up with a sour horse by trying to over think and micro manage. There are things with horses that you just go do, you don't practice you don't perfect you just do.
The best advice is the advice that you got on 'the nasty' forum, and what I see the nice people here moving towards....
STOP asking for advice about every little thing on online forums, you say you trust your barn owner, work with them and your trainer, mix with actual real people at the barn, observe, ask questions and copy, you CAN NOT gain experience here, because every interaction with a horse is different..
Take this picking feet up, no one knows, because we can't see what you are doing, what she is doing, and how the interaction works, someone actually there could help you in two seconds, because they can see what is happening.
Forums are a great place to share, share the joy of ownership, they are a great place to share ideas, to learn about new things, but honestly not such a great place for the total novice to try and learn stuff, because there is more than one right way to do things, everyone has their own favorite, some things work well, some don't, but you need to have a little experience to be able to sort things out.