While recent directives from the AG's office are a bit baffling and in contrast with codified MA law, I don't think that you have as good a handle on MA law as you think you do in regard to building an AR15--or the liabilities you risk.
On the Federal level, there are some very specific laws you will want to avoid running afoul of, for instance, if you are talking about "crowd-sourcing" tooling, you absolutely are NOT fluent with some of the more recent ATF findings/rulings on the topic. As I understand it, for this to be a personal build (and legal under Federal law), you must own the tooling personally. You may not use someone elses tooling. You are free, however, to sell the tooling to whoever you wish, at whatever cost you wish, on whatever timetable you feel is appropriate after you are finished with it.
Lastly--all legal jeopardy (of course) is contingent upon what can be proved in a court of law. If a charge can't be proved beyond a reasonable doubt, a case (theoretically) can not be brought against you, and certainly should not be won. What small fortune it might cost you in lawyers and legal defense while winning such a case is a seperate issue.
Enjoy your new gunsmithing hobby--but be sure that you don't get your ass in a sling on a Federal manufacturing charge, much less a MA "engaged in the manufacture of a prohibited assault weapon", because then Private Pyle, you will be living in a world of sh*t.