1. 5/5
2.
-bair>bair and some times more bairs
-bair>side-b
-utilt>bair
3. Diddy's bair is easily one of his best moves. For one you can do 2 of them in one SH which automatically makes makes it good. It comes out relatively fast with relatively low cool down. Great spacing and zoning tool as well as shield pressure. Retreating bairs are always good, especially because you can side-b back the other way while your opponent is shielding yours bairs. Good at edgeguarding, etc, wall jump bairs are also fairly fast and useful.
4. Not many tricks regarding this move, it's just an all around good move especially with spacing and zoning. If you catch a banana with it (and this goes with any aerial, the bair is just what aerial you usually do it with) instead of throwing the banana you will continue to bair if you input it until a certain amount of time has passed or you land on the ground. If you want to throw the banana then you must instant throw it instead to cancel that property by using the airdodge.