I like your designs, thank you for sharing them. It’s an interesting UX puzzle to think about.
1 was immediately easy to understand so I was drawn to it. Love the idea of the stereo send/returns. You could potentially make them pre/post configurable with the master section space. If you didn’t do that, I think making them pre makes the mixer a bit more adaptable. Another cool use of that space would be a 1 knob bus comp (though really a 1-knob bus comp might best be a 2-knob one with wet/dry for pushing hard and parallel processing so that might not work too well in the space). 
2 – I think it’s cool you did the diagonal jacks because it would make using right angled cables possible, which would come out of the mixer in a much more elegant way. they might need to be spaced just a tad more to make that perfect…I’d experiment with that if this ever came to fruition.
I do think in a small package like this a dialed-in one knob eq is a bit better use of space than 2-knob eq. And master eq
3–this one took me a second to get, but I like it! I think as @andrew mentions a combination of 1 and 3 would be nice. Maybe 1 or 2 of these channel linking pairs could replace the mono section on 1…the pan knob being replaced with the 1-knob eq. Or maybe just have these linking channels but with the stereo sends.
I do get a lot of use out of mute/solo buttons when I’m making music (so that I can solo and hone in on a certain sound). I do think the things your looking at are small enough where you could emulate solo with multiple finger simultaneous mute taps, so maybe to save space just mute would be best?
also, this is probably extremely niche, and no idea how the circuit it’d work, but I have personally tended to do stuff more back-forward mixing with m/s balance rather than around the l/r spectrum coz I think it feels better to me. could be cool if you could change what the balance control does with an internal jumper or something.