Hi Peter,
Yeah, the lens is wonderful, as it was on the original RX1. The menu structure isn't intuitive, but it doesn't take long to figure out. And, in my experience, the camera is so customizable, that once you spend some time up-front setting it up to your liking, you won't be using the menus much anyway. I'd give it a couple hours of intensive playing around (and eating batteries - battery life is a weak spot with the original RX1 and weaker spot with the new one) to really get it down and then a few days of shooting to change settings and custom options to really get it set up the way you like it. And then the interface is just fine because it's yours.
The sensor is technically better than those in the D750 or DF, but it's more similar in it's approach and look to the d750 than to the df. The sensor in the original RX1 was basically identical to the one in the D610 and D750 except that the original RX1 didn't let you shoot the raw uncompressed, so it didn't have quite the latitude of the Nikons. The new one does allow you to shoot compressed or uncompressed, but the uncompressed files are ENORMOUS with this higher res sensor, so I mostly shoot it compressed - I was always happy with the original RX1 files and these have that much more DR and are that much better at high ISO, even without shooting uncompressed. For an occasional landscape I might turn the compression off to get every bit of latitude I can, but that's pretty rare - I usually shoot compressed. It's an amazing sensor technically, but I still somewhat prefer the flavor of the DF files, despite it's lesser base ISO DR.
The colors I can't comment a lot on - the raw files are as malleable as you want them to be and I can get colors I like more than well enough. I almost never shoot jpegs, so I can't really comment on those or compare them between Sony and Nikon, although I think the general consensus is that Sony doesn't do jpegs as well.
And I don't have any idea about triggering external flashes. I pretty much never used even the internal tiny flash in the first RX1 and haven't even thought about using an external one with the new camera. These are extremely good low light cameras and, for what and how I shoot, I just rely on that and basically don't use flash. But, if you can get it set up with an external flash setup, your concerns about DOF should be alleviated - you can close the aperture down as far as you need shooting with a flash, no?
In terms of handling, I've used it both with and without a grip and I mostly use it with the Fotodiox grip, which is about $50-60 and well worth it. The camera is so small and light that shooting with it for long periods of time is not an issue. For me at least. The only question is whether you'd like it more with or without a grip.
The only other thing I'd mention is that for what you're talking about shooting, you might do just as well and save a bundle of money with the original RX1. I thought that was a wonderful camera and if they'd just set up the auto-ISO options to my liking (primarily an issue for the way I shoot on the street), I'd have never sold it or even considered the new model. The built in EVF and the tilt screen are pretty nice to have, but I was fine without them in the original. And that sensor, as you're aware from the D750, is incredibly good even if the new higher res version is technically somewhat better. My ideal small camera would be the original RX1 with the DF sensor, and a slightly wider 28mm lens. But nobody's offering that and the new RX1 is best for my needs. But I really loved the original and for everything but the way I do street shooting, I liked it every bit as much as the new one.
Hope this is useful... If I missed anything or if you have any other questions, let me know.
-Ray