1. There's nothing wrong with 5DMk2 AF. People were shooting Formula 1 races before AF, with 1st generation AF, 2nd gen AF and AF far more "primitive" than what the 5D offers. I have the 5D2. I have no problem using any of the focus points. If there is any hunting, it's because the sensor isn't on a contrast area. I don't expect miracles or for the camera to read my mind.
2. I haven't owned a Mamiya 7. I have had other MF rangefinders, though, including the Mamiya 6 and Bronica RF645. I was able to get pretty convincing simulations of my RF645+Portra 400 with the 5D2 and Exposure 2. Convincing enough that if i look back at the pictures, i probably don't remember which were film and which were digital. They're not exact, but close enough that i'm not reminded they're digital. And Exposure 3 is even better.
3. I used to like Luis Sanchis a lot. Haven't followed him in a while, though. I would imagine that stuff is Mamiya RZ with film. If you want to really shoot fashion, i wouldn't recommend a rangefinder. I've shot fashion and i've used rangefinders (35mm + MF) and would never dream of shooting with anything other than SLRs, especially if you want to do such 'tight' work as Sanchis. Having multiple film backs and closeup capability are two other features of the RZ that trump the 7 for this kind of work.
4. I agree with the 5D2 as the choice. And especially over a Nikon if you want to use primes. Nikon 50mm lenses have pretty bad, unpredictable bokeh. Not such a factor if you're used to closing down the lens as i see on your site, but it's something to be aware of. Of course, you could opt for the Sigma 50/1.4.... Same, though, with the 35mm focal length - the Canons are better than the Nikon AFs, except for the Nikon 35/1.8, but that's for APS-C sensors. Canon's 85s are also better. The inexpensive 85/1.8 EF has Sonnar-like bokeh. I sold my 85L and will be buying the 1.8. Not that the 85L is inferior. But, its size+weight made it too much of a 'specialty' lens for me. Now that i've stopped shooting models, i want an 85 for travel/environmental portraiture, etc.....
I don't think you're going to replicate the Mamiya 7+Portra aesthetic exactly. But, let's look at it a different way. That 'look' is essentially a combination of film, lens, and light and the interaction of those components. Processing is neutral. That's not really a formula you 'created' in a sense. You sorta chose it, or it was something you came upon and adopted. Who's to say you won't be able to actually create a look you like as much or more with the Canon? You will just have to be much more involved in that process, though. Photoshop is an amazing piece of software and plugins like Exposure get us much closer to the analog feel. You might just find something there that is 'better' than the Portra thing.
Regarding the claims that the Mamiya's IQ is far above the Canon's..... I've have all sorts of MF cameras, including the RZ, Pentax67, various 6x4.5s, Hasselblads, and Rolleiflexes. But, my digital pictures ALWAYS appear to be sharper than the film pictures. I'm aware that there's probably more 'detail'/information in a high res film scan from large film. But, apparent sharpness has little to do with that. And, the clean digital files are different from film - film has 'schmutz' in there with the 'detail.' You can muck up the digital files to get closer to the 'dirty' film. Not exactly, but convincingly enough that a print shows little difference if any.
Another thing to consider - when you shoot digital, don't use the white balance to make everything neutral. With film, you chose a daylight balanced emulsion and you accept the color casts and changes that invariably come when you have mixed light, or cool or warm light. Do the same thing with digital. You should shoot RAW, but choose daylight WB and see if that makes your pictures look less 'sterile.' That, to me, was one of the beauties of the work of guys like David Allan Harvey, when he was shooting film. The 'off' color temps.... With digital, people tend to clean that away. Unfortunately. If you shoot fashion in the studio, you'll probably want/need to keep it neutral, but outdoors?
Personally, i still prefer the look of film. But, i love digital's convenience, speed, responsiveness, etc. The one thing i still haven't adjusted to, though, is the FEEL of digital cameras. The 1-series is better in this respect, but the 5D2 still feels like a plastic consumer cam. I'd much rather use my FE2, sadly. You mentioned that the 1 series is too large, but i actually have the battery grip on my 5D2 to make it feel more solid. It helps. But, the downside is the size. I probably should just use a 1Ds, but don't want to spend that kind of money when i'm still trying to use film in all my critical situations.....