The front lens of the lens is strongly curved and has a permanently integrated lens hood, threads for screwing are not available on the optics. The cap is placed over the lens hood and held in place by latches. I don't see any options for attaching a filter, and I'm not aware of any factory solutions or products from other manufacturers. 

Yes. There is a slot in the housing around the ball head that allows it to go fully perpendicular to the sky. You can rotate the ball to capture any part of the sky you want. 

I've never used Hoya ND's, but I have GOBE, KERNEL, ZOMEI, and K&F ND filters. Out of all of them, I like the K&F the most because the colors look great out of the camera and they don't create a vignette around the edges like some of my other nd's which I hate. Because of the great out of camera colors, it gives you more to work with on post-production when you shoot on a flat profile. Also, I've used them during night photography on bulb mode to do light painting and the 4 - 6 stop ones work great for this. Overall, I think for the money the K&F filters are worth it. Awesome product! 

I would say...a qualified "yes." You are going to have a nice wide field-of-view and sharp focus. The 40mm objectives are not going to allow a huge amount of light gathering but you will have portability and that counts for a lot - a light, sharp and accurate optic is more useful than a large and bulky choice. Even if that means giving up some "reach." 

Hi, the K&F black soft filter 1/4 has the same effect as Tiffen black pro-mist filter 1/4, just difference in name , thanks.