Chief, the following is all speculation on my part. I think No. 1 above is probably true; not because of a lack of power, but of will. Starting a war is a daunting thing to do, and Buchanan's heart would not have been in it.
No. 2 I am not so sure Blair was correct. Given lincoln's desire to save the Union, the issue of slavery aside, with Sumter already taken, he could see himself as restoring the Union by attacking to recover Sumter, as he would not then be the aggressor. I agree he took great pains to sucker the Confederates into attacking the fort. Just as pickens had information from Washington, it is likely that Lincoln had at least some idea of the correspondence between Pickens and Davis, or at least of their mindset.
Finally, I wonder if Joe Brown had been in Picken's place, would he have waited for Davis, or even consulted with Davis, before taking the fort. If he had acted, then Lincoln would have the same circumstance without his manuervering to set it up that the South struck first. The end result would likely have been the same. Stan