I've sent you the preview file, which I just downloaded from KDP. I don't know how that will differ from what I uploaded, or why it should be different?
Of course here is the theory that KDP fixes the start problem at this point.
Do you think as a precaution they unpack the upload, remove the kindlegensrc.zip and put that back through Kindlegen which should give them the same file unchanged?
I doubt sending them the "fixed" file would help, because the problem is not the original error which is manifested as an inaccuracy in where it jumps to. Instead we now see a complete failure to go to the start position.
In any case I don't know how to send them the fixed file. The special split only gives me the two separate files, mobi7 and mobi8, and I don't have any way to reassemble them other than to put one back through Kindlegen which would make the error come back again.