I'm a diehard NatKitty shipper...I'm sure that Kitty's just better at hiding her feelings than Nat is, or maybe it's just that Barti decribed it very well for us inside his head...I dunno. I'm certain it would have gone that way if he hadn't died, but I supported them even after GE when he betrayed her...so maybe I'm just obsessive!
As for Farrar...I think Nat had a crush on her until in the middle of PG when he dismissed Barti (when he was a dying frog)...he sort of had a realization then that she was NOT A NICE PERSON! And that she wasn't someone who would ever understand him. That's why he needed Kitty and Ms. Lutyens!
(*is proud of speculation*)
Don't kill me J/J shippers!

MUSICA DIABOLOS
http://www.fanfiction.net/u/962826/
Nathaniel + Kitty Forever!
"It's two thousand, one hundred and twenty-nine years since Ptolemy died. I still wear his form. Who do you think's the lucky one?"
- Bartimaeus, Ptolemy's Gate