do the Magic Guys have to begin again from scratch every time they 'assault' a new model or can the previous work be transferred, in the form of pieces of code or at least expertise, from one camera to another?
No, the basic ML codebase stays the same and is very mature by now - i.e. integrating with Canon DryOS as far as setting props and memory allocation is concerned. The 7d1 has a unique catch though which might also apply to the 7d2: real dual digic-processing (and not just offloading the af to a digic).
That's the reason the 7d1 port didn't get started for some years as it wouldn't boot at all, and even now some features don't work at all. In general, my estimation is one year from "Hello World" on a newer camera to a release which has been tested enough so everything works w/o hiccups. *If* a maintainer for the camera is found, and that's not very likely since it's an unpaid job requiring months of man-work. Btw the 6d is unmaintained right now (but somehow working), and so is the 100d (not working at all).
and I thought I read here once, that the 5D3 was the mini 1Dx ;-)
I imagine the 5d3's frame rate is still way too slow for pro sports shooting and maybe wildlife - for these it might be a fraction of a second between the and perfect shot you sell and junk that looks like your mom shot it. I'm a flash user and really not a fan of high fps, but if I'd want to be sure to capture the moment I'd rather capture a frame off raw video with enough resolution for most print/web purposes.
Btw if you don't know it: Magic Lantern has not only raw video, but also a "silent pix" function which captures bursts of 60fps+ even on cameras which sd card cannot sustain high write speeds. It even filters out shots that are out of focus, the catch is that it needs live view and doesn't work with af but only locked focus.
The mention that this will be positioned as a Professional APS-C camera is what I hoped for.
Professional also means 1dx-like shutter cycles. Burst with mirror/shutter up in lv have the advantage of not eating through your shutter - but did you ever calculate how long your 5d3 lasts @max fps and 150k warranty cycles :-p ? How long a supposed 7d2 lasts @10+fps ?