Mark where you want the tap tower on the granite counter top.
Check for adjustment room both fore and aft, and laterally, on your fridge.
Make a pilot hole in the granite top, and transfer this MARK to the top of your fridge.
Pull the fridge out, and make sure the mark is centered on the top of the fridge, ( this may not be necessary to "center" it, as it will be hidden from view).
I would THINK, that the tap tower would be mounted ONLY to the granite top.
When the modifications are done, you can make a compressible foam collar to seal the gap betwixt fridge and counter top underside.
BEFORE, you do all this, make sure you can make a hole in your fridge top.
Be advised: you will have some warm beer lines running up to your tap tower, which can lead to foamy pours, so you'll need a way to keep the tower cool, as it will probably see some sunshine.
At least that's the way I would approach it.