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

The way u tell is to look at the end of you lens. There will be a 0 with a slash / through it and a number behind it. Match that number to the 58 of this neutral density filter. 

hello , we are the seller, you could use an adapter to step the 52 down to a 49 , there is a adater ring kit https://www.amazon.com/dp/B00JL4HGAI, 

There are 6PCS adapter ring included in this flash, here is the size:
52 55 58 62 67 77mm, as long as your lens size is one of them, then could be used,
Here is the way to check your lens size:
Your camera's lens thread size will be marked somewhere on the lens barrel or printed underneath your lens cap. This number is always preceded by a "?" (diameter) symbol. For example: ?52 = 52mm lens thread size.
If your lens size is none of them, please tell us, we may include the size you need but at this case, this item must be shipped from China so that we could pack with this adapter ring you need.