What I was trying to point out (and you obviously missed) was the multiple silly assumptions that your claims are based on. Let's just look at the doppelganger assumption. You think that I should worry that my girlfriend has been taken over by a doppelganger. Personally, I would be far more worried that my wife might find out.
Let's imagine, for the sake of argument, that my wife might have been taken over by a doppelganger. Why should I assume that a simple question like "Where did we first meet?" would permit me to detect whether the creature in question really was my wife? Perhaps the doppelganger has ability to answer such simple questions as well as my wife could.
Why should I assume that said doppelganger has murderous intentions? Perhaps it only wishes to obtain genetic material from me through interchange of sexual fluids for the purpose of producing more doppelgangers? The doppelganger might also have the added bonus of being able to change shape at will. Maybe I could request that it take the shape of Giuliana Rengifo and then I would probably enjoy the experience immensely.
If the doppelganger really were murderous, taking the shape of my wife (assuming it would also take on my wife's strength) seems rather foolish. I would easily be able to overpower it. Or, on the other hand, if it had not my wife's strength but rather super strength then even knowing that it was a doppelganger would not help me as it would easily be able to overpower me. In fact, by detecting it as a doppelganger, might I not just as easily make it feel that its cover had been blown and that I was a threat that needed elimination?
Your claim that I think about and solve all of these problems through the use of parsimony is overdone, I think.