Can't really comment on the gap back and top I'm afraid. I haven't backed the fridge onto the side wall like most, mine backs on to the driver's seat and the compressor is out in the open.
yer, fitting kit bolts to the front with no rear support other than the weight of the fridge itself. I've put a couple of steel brackets behind the fridge itself just in case the cupboard and frame snaps or the screws get ripped out of the cupboard... doubtful i know.
anyway, if it's of any use though here's a pic of the flush mount kit from underneath...
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fridge then sits on the cupboard floor and frame bolts to the front of the cupboard floor (in my case an mdf facia surround then the floor)