UR series and features

Hi. I bought an UR22 a few weeks ago and I am some what happy with it but are lacking a few inputs and there seems to be a little noise coming from the pres. Also, it’s not 100% stereo! There is a bit of leaking between the left and right channels when hard panning. So wierd. Anyone else notice this?
But, the sound quality is quite good so I think I might buy an UR unit that has more inputs. And I hear they have an AC power so there is less noise from the USB port. But here’s my question: both the UR44 and UR 242 have enough imputs (would like an spdf input, but what the hell) but there is no “mix” knob on both units. I need a direct signal when tracking guitars in headphones and a no latency tolerance here! The ur22 have a direkt input so I hav track guitar without going through my DAW. But I read the UR242 has a zero latency mode. How does that work? Is it really ZERO latency?