Based on your comments, I would not do the recommended tests either. You already have, and more. The webviewer test was the clincher. You have narrowed the issues to the NVR itself enough to satisfy even this old dog.
Generally speaking, Vitek is preferred by installers because it is cheaper and allows more profit. Also, their reps are very active, putting the product in front of their customers (installers). Their 960 product line was solid, but Analog is easier to engineer and the results are a known quantity. That technology is easily replicated by a number of manufacturers. IP as you now know is a much different ballgame. It requires R&D and then long term testing that not every manufacturer is willing to invest in. "I's" don't get dotted and "t"s" don'g get crossed. It should not be rushed to the market.
"DW" is manufactured overseas as well, but again, they are a very mature company with a robust R&D department. When they put their name on it, it has been thoroughly tested. Their biggest issue is on the sales side of things. They depend on Rep Firms (some are good and some are bad). Their factory people are really quite good. As for the 2 monitor concept, you can do that but don't have to. They have software that will allow you to display the video from more than one NVR at a time, but I think you are limited to a combination of 16.
March is fully mature product initially designed by and for IT end users. Logs, security, bandwidth throttling are all things they designed into their platforms when IP DVR/NVR's began. In the 13 years I have with them, I have returned no more than 5 machines. Things go wrong yes, but we are able to work with the factory and get them back up. But again, it is clearly more expensive. The feature set is worth it to our Financial customers who have to answer to the FDIC auditors.
Hikvision is a real enigma to most experienced security pros. They have been around for a long time, mostly as an OEM partner, making products for others, so they have the resources. They understand quality and reliability, but keep things simple. In just the last few years they have morphed into a formidable force in the western markets. They do lack a few things, most notably people, but they are ramping that up pretty nicely too. What they are able to do for the prices they charge is baffling. They are either using sub-slave labor or have a heavily automated magic factory somewhere. It would appear on the surface the only thing capable of stopping Hikvision is Hikvision.
You seem pretty savvy to me. Even if you do run across an integrator unwilling to demonstrate products to you without a firm commitment, I think you can handle it. A commitment to a good integrator, even the one you have, will go a long way to solving your problems no matter which manufacturer you select. Just be firm and fair.
Good luck and please visit again. We always enjoy hearing how things turn out. We learn from it.
Edit: The main reason I mention DW is that they include the Vitek camera software in their NVR's. The list of manufacturers that write Vitek code into their recording software that I trust is short, but DW is one. If your cameras are working in the webviewer and you like the results, I would not be so quick to trash them. Money is money.