What's the best MOSFET for switching small currents?
"Why was everyone saying this would be so difficult? Am i missing anything here?"

Does it bother you at all that the BSS84 used in your model has a leakage spec that is about 2X the 50nA you are trying to switch?  

Single MOSFETs are designed for low Rds for die size, not so much for switching extremely low currents (although tcrosley found one that will almost do what you want). To switch low currents like this, one would use a low leakage analog switch. For example something like DG2012 might work.  

Remember that all models are wrong, and it is up to you to figure out if it is right enough to be useful.