I can't comment on the first boss fight as I haven't seen the chest bugged. On the boss fight vs the 4 mobs we did have the chest remain locked the first time. Doing a bit of reading we found someone saying you needed to take them down in a particular order. And forgive me if I don't recall the names as I don't have that in front of me: Consular first (left most), then Sentinel (second from left), then work the other two down to die at roughly the same time. I can't say for sure that is the solution, but that sequence allows us to open the chest. As for difficulty, we were able to finish this Flashpoint with 3 players, two 23s (tank & dps), a 24 (healer) and a DPS companion. It helps if you save and activate the medical droids just before the fight.