For the Nikon 7200 you need a filter in the size 67 mm, this also applies to all lenses. 

1. The other end is threaded, and filters can be superimposed by purchasing filters of the same caliber (for example: 58 black soft filter, superimposing 58 ND filter on the back) 

1. The outer diameter of different diameters will be different, you can superimpose the filters, you can buy the same diameter filter superimposed (such as: 58 black soft filter, superimposed on the back of the 58 ND filter) 

Yes, I believe you can with any trail cam. There are several videos on YouTube. You need a 4.0x1.7mm plug. I saw one video that explains a direct hard wire. After watching some videos, I decided it was easier and about the same price to buy the Stealth Cam 12v external battery pack with solar panel. I paid $50 at Academy Sports online. Plus the 4.0x1.7mm plug from Amazon. But if you like to make things, there are plenty of videos on making your own solar battery packs. 

Yes, it is threaded on both sides (male and female). This is a very high quality filter for the price.