Swanpride wrote:

The only thing I would like to know is if Mycroft knows about Mary, and if he does, when he knew.

100% agree

SusiGo wrote:

And he would have researched her thoroughly.
If Mary has been planted there - which is entirely possible given her special "talents" and the fact that it would be an enormous coincidence that someone like her meets John just like that - I suppose it was by someone else. Which of course may depend on the question if Moriarty is really dead or if someone uses him. 

Wouldn't be the first time Moreiarty planted an assassin near John and Sherlock.

So here is the begining of my first attempt at fanfiction.  I started right where The Last Vow ended.

This was the second time John Watson thought he had lost Sherlock forever, however that did not make the loss any easier.  He supposed now he had Mary, wonderful, beautiful, lying Mary.  Who would, if not stop him from breaking into pieces, perhaps do her best putting him back together.  He stood looking into the sky, even after the plane disappeared.  He didn’t know Sherlock did the same, looking out airplane window, eyes fixed to the ground below.  Both men still felt an electric sensation in their right hand, from their last touch.
John knew this was wrong.  Sherlock Holmes, the best and bravest man he had ever known, gone.  He would never be able to forgive himself.  He is the one that should have shot the terrible shark that was Magnussen.  It was his wife and child that needed protection, they were all safe now.  Safe from Mary’s dark past.  Sherlock had thrown away everything for John. The Work and possibly his life thought John with a horrible itching suspicion that now took over his mind.  This was definitely worse than before.  Never knowing.  Before there had been no doubt he was dead and wouldn’t return.  John had found Mary and continued on with his life.  But now he would always be looking over his shoulder.  Tall dark haired men in long coats would make him give a second glance, for he would never stop hoping Sherlock would find his way back to him.  He had come back from the dead once hadn’t he?
Mary stood somewhere right behind him.  She didn’t touch him, or tell him everything would be alright.  She would let him grieve, then expect him to start moving forward.  Mary was just what he needed, in a world without Sherlock Holmes.
“That is not possible. That is simply not possible.”  Mycroft spoke agitated, while getting out of his car.
Taking a step closer to the taller man he asked, “What

Since they are already useing Martin real wife, why not get his older sister Laura Watson?  That would be cool if she could act.