Interesting thought. 
But I think an ordinary fangirl doesn´t have the means and equipment neccessary to set up such huge plot... so here is my additional theory:

Anthea, Mycroft´s secretary, is a secret extreme fangirl of Sherlock. She became Mycroft´s assistant with the specific goal of seducing Sherlock eventually. That´s precisely the reason she never looks up from her Blackberry - it contains some hot pictures of Sherlock from secret cameras set up in 221B Baker Street and it´s a lenghty work to see them all and to drool over them (and of course, to miss even one of them is impossible for such a fanatical fan). The further proof of Anthea´s secret fangirl-dom is the fact that it was her who tenderly took care of Sherlock´s coat in his absence. The day she dressed Sherlock in TEH was probably her happiest time ever - and she did it with such affection, oh.... 

So, of course, when Anthea discovered from Mycroft that Sherlock is being sent away from the UK indefinitely, she lost no time and set up the "Moriarty´s return" using all resources that she had at hand as a member of MI6. 

Oh, thank you, Marta, for reminding me! How could I forget that!