Well, obviously the purpose of phase 1 is to prove the newly built aircraft safe to operate while risking as few lives as possible. An observer pilot is not allowed by the AC until the initial flight test program, with all the maneuvers and at least 8 hrs flown by the builder pilot, is complete and documented and has nothing to do with what I propose. If I intended to be an "observer pilot" I'd have no business being along on the first flight anyway, even if I did build it and know the airplane better than anybody.
The purpose of the additional pilot program, as set forth in the opening pages of the AC itself, is to cut down on the risk of an inexperienced pilot crashing his brand new plane early in the test flight period. So, taking along a very experienced and highly qualified pilot, who happens to be a CFI, with a ~20hr tailwheel builder pilot - how is that abuse of the additional pilot program? It is exactly what the program is for. If my insurance didn't mandate a CFI, I might well want to use a qualifying pilot per the additional pilot program for the first flight anyway.