Select MSZIP in the compression type of CABPACK then the file should have near the same size.
The step with a hybrid CAB is not nessesary.
When i did the downgrad on my scope i have tried out different things.
This step is only needed if you need the latest firmware on the scope with USB Boot (issues with different dll versions are posible??)
TO downgrade it must be enough to replace these two file with files from 2.35
infiniiVisionLauncher.exe
infiniiVisionCore.dll
after a reboot it should be possible to load the 3000XSeries.02.35.2013061800.cab
I have to test this on my scope again but this should be the easiest way and should work.
And while downgrade there should be no infiniivisionStartupOverride.txt on the usb drive