You won't know what they'll want until you ask, they might tell you over the phone but then no guarantee you'll get the same inspector when you put the Notice it. In modern houses they typically carry on the strip under the garage door anyway as it makes building the footings easier.
Silicone everywhere for me, it won't look out of place provided the colour is right and it's done neatly. I would question why wouldn't you seal between the floor and the walls/pan on a floor that's likely to get wet.
But you've got a loft leg every 600mm in every direction - correct?
TBH if I was forced to use loft legs I would probably have a bit of timber around the perimeter of the boarded space but that's because I think having the boards unsupported around the edges is a bit crap. But then I would...