A: The Camco Propane Cylinder Gas Gauge and Leak Detector # CAM59023 will fit on your 100 lb tank if there are threads on the outside of the valve. Most tanks have both outside and inside threading, but if you only have the inside threading then you will need an adapter like # 204128-MBS.
A: No, it just detects leaks.
A: It does not restrict the gas flow to a 5 burner grill, so it should be fine for your application.
A: No, it will tell you if you are loosing pressure.
A: This is a leak detector only. It will not stop the leak.