Well, so GarminCure3 did its job well. It makes your device enter the mass storage mode unconditionally. Now you can flash the original firmware only by putting your device into pre-boot mode.
In the GarminCure3 [Only registered and activated users can see links. ] it's written the following:
I really hope lolypop000 will help us with that. I've seen here a few similar kenwood devices which have working normal pre-boot mode.
For example read [Only registered and activated users can see links. ].
This is a success story where GarminCure3 and my removing of region 0x66 did help. And the device looks quite similar to yours.
It was successful mainly because Toretto was able to enter pre-boot mode.