Try setting your D5200 to "Matrix" metering, and make sure the SB600 is set to TTL, not TTL-BL. From my SB600 manual: "Standard TTL - The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject." TTL-BL is Balanced Fill-Flash - "... well-balanced exposure of both subject and background."

Happens to me all the time. There are two rings on a circular polarizer - one spins to polarize, the other screws into the front of the lens. Put a rubber band around the screw ring to get a better grip on it. Same principle as a unscrewing a tight jar lid.

No one who has ever pulled a lens out of a backpack and shaken off the broken bits of glass that used to be the UV filter would ever ask this question.

I like to do what they call a "museum mount", that is I mount say a 6x9 print in the middle of a 8x12 foam core (both ratio 2:3), mat it and put in an 8x12 frame. How would that work with this Crescent adhesive foam core? Wouldn't it leave a sticky area around the perimeter of the print? Not sure I'd want the mat to stick to the foam core.

I mount photos on foamcore using a product called Bainbridge StudioTac. It's a cold drymount tissue, no muss, no fuss, seems to hold a tight as a hot drymount. It's kinda hard to find, I usually search the internet for a mailorder source.