You are proposing that Brandon didn't really rule out being able to go back a year by putting on his Aes Sedai hat and "assuming" a number that was smaller than the actual number to make the math not work out. However, Brandon said



He says "I just laughed...c'mon lets run the math on this." Not the response you would expect if the notes indicated that this is what happens (remember he has a lot of ToM written, and knows how the series turns out, including the reveal of Asmo's killer).

Second, he says "I think that if you did that to the Pattern the ramifications would be so dramatic you'd see the Pattern unraveling hardcore at that point, it's like balefiring an entire city." This is the ramification of balefiring ONE person back ONE year. There was something like 100-200 people in the fortress IIRC. 150 people back 1 year is the same as 1.3 million back 1 hour (150 people*1 year*365 days/year*24 hours/day = 1,314,000 people hours (the SI unit for balefires - or BFs for short )) and the same as balefiring It would be the same as balefiring 5,256,000 people back 15 minutes (the more realistic assumption for how far the strongest channeler in the world can balefire). The Pattern may have unraveled under the strain of that many people being balefired back 1 year.