Re: time to hire up a coach?
Summon, like Rerun has mentioned, compared to other players, who experienced the same kind of long injury-break, for example Kim Clijsters, Maria proves to be much more struggling to get back into form. There are benchmarks out there and Maria's not doing great for that matter, neither is she doing bad, just so-so.
I'm not saying at all that there's anything catastrophic in what we saw so far and I have no doubt that new coach or not, things will go improving. The question is how fast are things going to improve and if it's going all the way to coming back as GS-winner/World #1 material.
There are two aspects to mental toughness: fighting spirit and confidence. The first one is intact but the second one is not really there so far. That's where a coach can help a lot if s/he is the right person for the right player.