I wouldn't want you putting your feet further into the stirrups though, until you invest in a proper pair of riding boots. Those boots could easily become dangerous and leave you strung up by the stirrup should you come off.
I say question your instructor. They're not gods! I question mine all the time, and she appreciates it. Often she actually learns things from me when we go into these discussions. Possibly your instructor is strapping all these gadgets onto the horse because she feels it will make her seem like she's is doing more, faster for you and the mare, rather than going about it the correct way of driving from behind into a steady and elastic contact. I would be worried that she'll start to learn that sitting behind the vertical is a good way to evade pressure, and then you will wind up in all sorts of trouble if later on down the track you become a more advanced rider and want to learn how to truly collect a horse. Having them behind the vertical and sucked back at the wither just doesn't cut it unfortunately. It may look 'pretty' at a pony club level, but out in the 'real world' you'll get struck down for it. Just the way it is sadly, with horses there are just no shortcuts.
Your position really isn't that bad. You're tidy, nothing sticking out and no drastic leaning/dropping to be seen. Only those couple of small details like your feet which as I said above, I wouldn't change until you're riding in good riding boots, and sitting back a smidgen.
I love that you have a perfect line between your elbow, wrist, hand and bit. This is one of the more common errors that may be seen on people asking for critiques, so you've got that down very well