Just a thought. when you leveled the frets you say the neck was not parfectly stright, try
did you take a note of the gap there was in the middle of the neck?, try using the truss rod to get the gap you had when leveling the neck to same as when you have the strings on, the frets should be level when the neck is in that position, never know it might work, also when you were using the crowning file are you sure you did not lower the tops of some of the frets?, this was my major problem when leveling and crowning, I always went too far with the crowning file, I now shape the fret with a crowning file, and use a micro file to finish the top of the fret, it seems to work best for me, one other thinf I do when leveling a neck is from the 12th or 17th your choice, take a little more off the fret tops if you think about it, you will always need a little relief in the middle of the neck, this causes the two ends of the neck to be higher than the middle, which can suse buzzing, so if you remove a little more on the bridge side of the neck it will counteract this.
Eddy