Well, the love interest was hardly superfluous in that it's how Sherlock worked out how to unlock the magic phone. And the fact that he'd fallen for her was supposed to give her betrayal of him that added kick at the end.
One of my minor niggles is the plane-full-of-dead people was completely balmy and wouldn't even get much credence in a Catskillz YouTube video. Although a great image.
And of course th phone was a McGuffin that started to creak quite badly towards the end. Why couldn't they destroy it? Because it might have information on that would save lives. Er, okay.
Oh, and whenever you see an unrecognisable corpse in crime fiction it's never the person everyone says it is. So sexy tart returning wasn't much of a shock.
But these are minor quibbles as it was genuinely pretty good. And hats off to the actress for filming that scene with no clothes on (or, at the very least, strategically placed tape). That must have been quite awkward.