That'd all be correct if the unit still had HWID 1517 and was responding to 2797's boot.bin/ldr.bin.

Nothing happen!
Well Adrian, what i think has happened is that you have managed to flash components from the 3597's ldr.bin [boot.bin] to your 2797. This has changed Boot BLK and X-Loader HWID to 1603 while the System software is still 1517. Naturally, because the wrong 'starting blocks' to initiate the X-Loader are now in the unit's rgn43 and the wrong bootloader is in rgn5 there's now no ability to properly initiate the boot process and even if it could, the [presently] 'mismatched' system software in rgn14 is not going to be executed. At least it isn't dead and it does try to boot.

I doubt flashing the 3597's rgn140 to your unit caused any problem at all and of course if the boot problem can be repaired it will then be easy to flash back the original anyway. I believe that there is more than one likely reason why 'nothing happens' when you tried to write back the 140 backup from microSD card:
1) The folder '1517' is ignored because of the HWID mismatch; or
2) Even if '1517' is recognised on the card [but it's unlikely imo] because of the system software still being 1517, the 'now-foreign' 2797 ldr.bin cannot work anyway; or
3) Because the boot process is not even starting, nothing is able to be 'looked at' on the card as the card is not even being checked.

If it's 3), then you have a big problem. But if it's 1) or 2) perhaps it can be saved.

So, to have any chance of repairing this unit, the original 2797 'boot' components need be flashed back to rgns 5 and 43 to overwrite the wrong 3797 components you have now flashed there. However to have the correct info written to them you do need to use the ldr.bin from 3597 in '1603' folder to wake the unit. I can supply copies of both those regions from a US/EU firmware 2797, BUT there is a problem to flash them because although we can dump them they are protected from being written to by xrgn command or even in preboot as an RGN file. To write to those regions the ldr.bin/boot.bin must be modified to allow that and i don't know how to do it.

It can however be done, because that method was used to repair a 1490 hwid 0972 flashed with ldr.bin from 2x5W hwid 0827: [Only registered and activated users can see links. ]. The solution by our brilliant software expert kunix is in Post #33. You should hope that he will post here to help you too.

I probably don't need to explain that i'm not a programmer. My layman's explanation as to how the boot process works in modern Garmin nuvis is maybe quite over-simplistic and perhaps it's not even totally accurate. kunix may choose to give an explanation as to how these ARM based units start which will make more sense to you being you're a systems programmer.