Mate, you owe us nothing. We like to try to help ppl, most of us got help here originally ourselves. Our payment is simply having our efforts appreciated. You clearly appreciate the attempts, even when we are just fumbling around in the dark like i am with this problem of yours. I'm hoping others with more software moxie than myself can join in to help.

Your theory that the place to press for preboot may have changed is interesting, and i wouldn't wish to discourage you from trying all over the screen. However, even if it is now in a different spot, i don't think it will be for the reason you've hypothesized. The logo/splash screen normally seen on starting is from a PNG contained in rgn16. Both the units' PNG images have picture size 800 x 480 pixels and are identical. The 5" 3597 has proportionally better resolution than the 7" 2797 is all. Not only are the images themselves identical but so are the 16.bin dumps from 3597 and 2797, and by that i mean not only identical in size (3,145,728 bytes) but also their hashes match [MD5 70B10413696B99E0617AD78F7E289FF5]. So the logo image is not the source of the white line problem at all and nor is it responsible for denying preboot, sorry. Even a unit that has no logo image in rgn16 will still enter preboot in the usual way.

I'd keep trying over and over to reach preboot as you're doing, especially in the top LH corner, but note that just unplugging/replugging the usb won't work. The unit needs to be fully OFF each time you try. If you can see the bootloop's happening in a regular cycle you may be able to time your attempts in synch with that by holding the screen after it turns off and before it starts again but i've never tried that so don't know if it will work. I've always forced a bootlooping unit fully off by holding the power button and then re-powered it by plugging in the usb while re-pressing the screen.