On a B787 on approach, with the speed window closed and when in VNAV PTH, will flap selection always automatically reduce the airspeed to flap min maneuvering speed?
No, it won't. It'll happily reduce the speed to the top of the amber band. The autothrottle (*nervous tic*) system on the 787 - and all Boeings, really - is pretty rudimentary. As are the autopilot and horizontal / vertical navigation modes. The 787 will do wonky turns, crap holds, weird offsets, aggressive bank angles and fly an ILS that would fail an instrument rating. It always hands the aircraft back to you slightly out of trim, and as you've already noticed, the FMAs aren't really helpful.
Unless it's an ISA day, VNAV will always put you in the wrong place on the profile for an idle descent and the only saving grace you get on the 787 is the VSD, which tells you in advance that it's going to cock it up.
You still have the joy of "HOLD" mode to figure out, which isn't satisfactorily explained by anyone.