I apologize for my delay in answering.
1. Correct, DH2G is incapable of 120hz span modes. The MViewer MV103 can but it will have a bit of pixel noise. At least my demo unit did. Why you need to split it out with a hardware solution is because Nvidia doesn't accommodate users with a dual display span mode. Nvidia Surround has never been very accommodating as far as span mode. It's been three displays or nothing. This is why I've been using AMD as my primary gaming video card since around 2011 because AMD can do 2x1, 3x1, 4x1, 5x1, 6x1, 2x2 and 3x2 span modes depending upon port count available.
2. Correct, what the DH2G, Zotac adaptor and the MView all do is hardware spanning.
3. My information on Nvidia 3D Vision is archaic. the last time I did 3D Vision was early 2012. I've since then sold off my GTX470 cards so I have no way of testing accurately Nvidia options with current drivers. I'm going off of almost 2 year old recollection. If anyone on the internet wanted to send me a Nvidia card so I can help people with projector questions related to Nvidia I wouldn't turn them down, but I also don't expect anyone would be that generous either.
4. I see no merit in hardware edge blending. The price point simply does not merit humoring it. I can always help you with Nthusim configuration should you need assistance with edge blending.
If you have any direct questions I'm always available on the Nthusim site 24/7. I only check in around here about once a week anymore, because of the hostile marketing atmosphere around here driving me away. All I can say is there are those that help others, then there are those that advertise.
-Brad