VEVOR 8FT Sliding Barn Door Hardware Kit, 330LBS Heavy Duty Barn Door Track Kit for Double Doors, Fit 3.7-4.3FT Total Wide and 1.3"-1.8" Thick 2 Door Panel, with Smooth & Silent Pulley (I Shape)

It is recommended that the track needs to be twice as long.

They just have different hook shapes, but the functions are the same：https://send.now/a/ZCN

Question should be answered by the manufacturer. But 16inches is standard.

The holes are 16 inches apart, yes.

Yes. I woud use this for exterior. The metal is solid and the moving parts are made of solid plastic material. I have a similar system that as been outside in all weather conditions for 7 years now with no loss in function. Even the metal parts are only just showing some minor rust.

I have not used this hardware outdoors. It has a very good coat of paint which I believe is powder coated, so it should do well in the elements but you can expect that the bearings will eventually get corrosion in them. A good maintenance plan for lubrication from time to time would be a good idea.

Yes, you can trim the track.