* You got me curious, so I did a little more digging. I have a pair of Jackson 5362. I checked my folder that contained the purchase receipt and instruction booklet; I also found the straps there (I've never bothered using them). Neither the booklet nor the Jackson website mentions the straps.
* If you do a Google search, you will find two unboxing videos for the Jackson 5320. In one video, the straps do not appear. But in the other video, when the host unpacks the boots, the straps are attached across the top two rows of hooks; tongues are in the normal position; straps across the fronts of the tongues. So apparently the straps were used for shipping (at least on some shipments). They weren't installed when I got my boots, but the fitter could have removed them to inspect the boots; the straps were tossed in the box.
* I also tried the straps as you have used them: tongues pulled out; straps in back of the tongues, attached across the bottom two rows of hooks. They do keep the tongues propped open; but the tongues on the 5300 series stay propped open without aid of the straps anyways.
* The straps apply mild compression across the throat of the uppers (more compression in the shipping mode than in the propping mode). But the throat is flexible, and any widening in that area is readily taken care of during lacing. There's no way that the straps (in either mode) would affect the fit of the heel.
* You didn't answer the questions I asked in my Reply #4.