Solar Panel Roof Drill-Free Corner Bracket Mount for RV, Boats, Caravans, Marine, Motorhomes - Set of 6(Corner Bracket)

I have four 25 watt panels do I need a set for each panel?

how much "clearance" (empty space) is there between the bottom of the panel and the roof, once everything is in place?

What is the thickness of the brackets?if I mount to tanneau&remove panel,can it still flip back??

What mounting adhesive/sealant is recommended by the manufacturer for installaton on an aluminum trailer roof? (I'd prefer not to drill)