Personally, I'm suspecting that the taxi driver may be the Mr. Outside himself. He's supposed to be old, and the driver certainly looked old.
More over so, Takizawa's phone kept open connection to Saki's, and I'm sure Mr. Outside would have access to all of the numbers' phone records and probably got curious about her involvement. Furthermore, weren't they having problems finding a taxi to get on in first place, yet we conviently find them riding one afterwards.
I'm sure this is just wild speculation, but that's my thoughts.
Also, I had expected X to die by end of the TV series, given how two of the three numbers had died off in the TV (as implied by the OP). Guess that death's reserved for the movies.
And finally, yes it's true that numbers will be killed once their balance hits zero, but Takizawa's balance is not zero YET. Remember, as Juiz had explained before: in an event of complex requests, they will be processed in smaller sections (and gradually decrease the balance). So until then Takizawa's still alive. And it's quite possible that he'll become the King before the balance hits zero. He never specified when to stop processing the request either, so even if he becomes King there's still bound to be some opposition, so any balance remaining can be still used to preserve his 'King' status.
...just won't stop editing, lol
I realized that Takizawa made comment about how he'd want to be the same person he was when he and Juiz meets could actually be interpreted as yet another request; I think that implies that once he and Juiz meets, under any circumstances, the brainwashing he recieved later would be undone.
