Whether Lincoln maintained troops in Fort Sumter, or abandoning the post, was not a "principle", it was a tactic in support of a great principle: preserving the Union.
The "Rubicon" was Confederates' military assault on United States troops in Fort Sumter, just as was, for example, the Japanese attack on Pearl Harbor.
Preserving the Union was the great principle against which Lincoln judged all his actions, from defending Fort Sumter to freeing slaves under Union Army control.
How you can get so much obvious pleasure from distorting and mocking that, I'll never understand.
Well, at least keeping the Virginia portion of it. Again, it's sounding either like a bit of a bait and switch, get Virginia to abandon this secessionist talk and then later go after the friends they were supporting, or an abandonment of this idea of "Preserving the Union" by keeping the most valuable state and letting the rest go.
Is there a third explanation? It appears to me that the strategy must have been to either temporarily mollify the Virginians, or to permanently mollify the Virginians.
Your take on this might be interesting. I'm not seeing as to how this can be portrayed as "principled" any way you go about it.