That is a neat device and just cheap eonough to give it a go even though I have no need for it. Might be great for silent practice. My wife thanks you

A lot of questions in one

-The Ultra-G won't replace a power attenuator, because, as spaceatl said above, it doesn't put any load on the amp, so you will still need to put a load, connected to the Ultra-G's direct link to cabinet. Put another way, the Ultra-G goes between your amp's speaker output  and the speaker's input.

-If you want to use Ultra-G's cabinet emulation (or IR's) , you'll want to disable mic/cab emulation on the amp, yes, otherwise there would be a redundance.
-The Ultra-G will accept both signals, the one from the direct out OR any one of the speaker outputs.
-The Ultra-G does go in between the amp and the cabinet and delivers a balanced, mic level signal at its XLR output, while also letting the amplified signal from your amp pass thru, directly from input to "direct link to cabinet"
-The "output to mixer" can be fed directly to anything that will accept a mic level, balanced signal, be it a mixer, an audio interface, snake, splitter, etc.
-It will give you those tube tones from the amp, so you can use them with your IR's for cab/mic simulation.

Now I have a question for you:
-Why do you need the Ultra-G for? The DT's direct out already does everything you want the Ultra-G to do, and does it better, I'm betting. The mic/cab simulations are really good, better than many (not all) IRs I've tried.