>> First reason comes out of one of the responses from etabeta to NLS. As etabeta writes, "sub-builds" are not possible because, "if you don't build the whole tree, you could break some dependence".
That's not a reason. Reason must have purpose to serve, goal to achieve. Your goal is to make combined build, and the reason is: you have to make combined build if you wanna make combined build. Your reason and the goal are the same, so it's non-sense, you have to tell us why do you want to make a combined build in the first place, what purpose "combined build" is supposed to serve?
>> Second, it doesn't look good if MAME is not able to provide a unified build while at the same time you can find it at the blog of an independent developer.
What purpose is that reason supposed to serve?