3F, I think that if we were all trying to agree on one definition, we would soon start fighting over that. Every debate on WUS about that resembled a brawl of troglodytes fighting with clubs and stones - while I doubt that would be the case here, I simply know the extent of possible disagreement.
Generally, it's sometimes hard to tell in-house from generic, because there's a ton of things in between. Exempli gratia: Aegler movements made for Rolex- in-house, because used by them only? Generic, because supplied by a third party? Well, fuck. No easy answer to that, believe you me. The closest to a "middle ground" that I've ever seen was the concept of "reserved" calibres, which means that it's either made by an external supplier for one manufacturer, or the entire works built on a generic base are so far from the regular generic thing, that it's hard to say "but it's the same thing." As to the definition, my take on that is somewhere between "can have generic components" and "designed from scratch." Obviously not every component has to be made from scratch, that would be ridiculous- stuff like mainsprings, hairsprings and balance wheels are often made by third party suppliers; only a handful of manufacturers makes their own hairsprings, let alone designs the alloys used for that.
Food for thought: Habring2 rattrapante movements. Technically a 7750 base and gear train, but the rest can't be found anywhere else but in their movements. Generic? In-house? I'll be fucked if I know. Guess "reserved" solves the problem, to a certain extent.
What I find very pleasing about that brand's attitude, is that they don't really make much of a generic vs. in-house claim. They say "we took this and that from the generic movement, we did this and that ourselves." No claim, no problem, and in a not-so-black-and-white case like that, they had a good sense not to make a claim- the good sense that ML (in my opinion) doesn't seem to have.
Generally, what has to be remembered, is that generic movements are delivered to manufacturers in various degrees of completion. Sometimes it's a complete movement that only has to be decorated, sometimes it's a raw (raw meaning pure brass with no finishing) base plate, and should the manufacturer wish to redesign the bridges and (rarely) the balance cock, sometimes also without the bridges.
How "finished" are the ebauches bought by ML? I don't know. I'd be betting on a "fairly raw" condition. I mean, to accommodate a module to drive the complications directly connected to the gear train, a stock gear train and its pinions may be out of question. So then again, probably also the amount of generic components supplied depends on the complications intended for the movement. And, of course, how the manufacturer designs a particular complication.
I think it wouldn't be a bad idea to ask ML themselves about how in-house is in-house in their case, but I'm getting the odd feeling that they wouldn't be happy to reply.
As to the similar engineering structure, you can't avoid certain similarities. In chronographs, that would be the clutch- there are generally three types of that, so the choice is limited. A bit like with escapements, if you ask me. So is the case with column wheel vs. cam-switching. To me, as long as it's built on a completely unique base plate, the rest of the engineering can stay similar, since one way or another, it always will be - after all, as the principle behind the way something works has to remain the same, and that's the limit of the room for creativity. For example, I've seen a lot of movements with a gear train designed to accommodate the biggest balance wheel possible, and the first of that kind is the Zenith 135. Does that make the movements from other makers, using the similar solution, any less in-house (as long as they weren't made only to be supplied to someone else)? IMO, no. Copying solutions has always been the case, so that's not really a factor in determining whether something gravitates towards generic or in-house.
You cannot explain away a wantonly immoral act, because you think that it is connected to some higher purpose.
Captain Jean-Luc Picard, Star Trek: The Next Generation