1. It's good.  It's probably not as robust as some of the military stuff I've seen but our demo units have been banged up quite a bit and have faired well.  

2. The glass is better on the D750.  When we started carrying the D series scopes I got to look at the D740, D750, D760, and D790.  The D750 and D790 both had a noticeably better picture IMO.  Night Optics claims 25-30% better light transmission with the lenses they use and I wouldn't say that's wrong though I had no way of measuring this myself.  The body is mostly the same except that the turrets are much better on the D750 and D790.  There is a much more tactile and defined click when making adjustments with these scopes.

3.  Unfilmed is better.  Whether you get Green or White is up to you, personally I prefer WP.  From what I've seen of Unfilmed tubes they perform better in every way.  Not to mention they are recoil rated up to .50 which means you can put it on just about anything and not have to worry about it.

4. I doubt it.  I know for sure we would not be able to offer this as an option.  It would take a lot of effort to pull tubes from both systems and switch them around.  

5. Yes, and I liked it a lot.  I liked the 6 power versions slightly more but both the D750 and D790 were excellent systems.  

6. Unfortunately the only pics I have are of filmed Green but you can check them out on TNVC's website.  I do have these though.  

MDH Barrett .50-6 Chip embossed 2 by Highway0311, on Flickr

MDH Barrett .50-0499 by Highway0311, on Flickr