I was struck by one quote from Lincoln's first inaugural address (emphasis added):

"The power confided to me will be used to hold, occupy, and possess the property and places belonging to the Government and to collect the duties and imposts; but beyond what may be necessary for these objects, there will be no invasion, no using of force against or among the people anywhere. Where hostility to the United States in any interior locality shall be so great and universal as to prevent competent resident citizens from holding the Federal offices, there will be no attempt to force obnoxious strangers among the people for that object. While the strict legal right may exist in the Government to enforce the exercise of these offices, the attempt to do so would be so irritating and so nearly impracticable withal that I deem it better to forego for the time the uses of such offices."

In other words, as long as they rendered unto Caesar and didn't take his stuff, Lincoln was willing to abandon all other federal government functions no matter how constitutionally mandated. This seems like secession in all but name.

Remember also that the casus belli was that Fort Sumter was supposed to be handed over to the Confederacy, but the federal government refused to.

Both seem more consistent with a power theory than a slavery theory.