If the rebels actually advance on Washington
How does Lincoln being scared more result in no amphibious attack?
Also what do you mean by "brittle"?
Also thanks alexcoppoe for the scenarios.
If the rebels actually advance on Washington after 1st Bull Run, they will be defeated, and by forces under the command of (presumably) Scott and McDowell; numbers and logistics are all in the US favor.
However, this gets us through 1861 and into the spring of 1862, with, presumably, either McDowell or McClellan in command in the eastern theater, facing (presumably) JE Johnston or Beauregard.
McDowell, with forces that are 3-year volunteers (rather than 3-months) is likely to argue his strategy of an overland movement will work, which gibes with Lincoln's desire to keep the US forces on the inside chord; McClellan, if he is in command, may want to argue for the amphibious movement, but Lincoln - with the example of the 1861 campaign - is probably going to say no, go overland, river line by river line, which certainly makes Johnston's or Beauregard's task that much more difficult.
"Brittle" in the sense that southern nationalism, the southern nation state, and the general state of the rebel government's institutions and domestic resources and internal politics were all much less resilient than their equivalents for the US, which is born out in (for example) the internal opposition to the Richmond government, which ran the gamut from the southern governors refusing to release their state forces and militia to "conferate" command to outright armed rebellion in locations as diverse as western Virginia, eastern Tennessee, western North Carolina, and Jones County, Mississippi.
Despite some of the mythology, southern society was far more divided over the Civil War than the north and west ever were; the reality that roughly 100,000
white men, of military age and residing in southern states in 1861, ended up serving in the US forces makes that clear, and historians ranging from Victoria Bynum and Richard Current have done great work laying it out ... add the white loyalists to the men of African ancestry (free or enslaved) who volunteered for the USCTs in 1863-64 (roughly half of the 180,000 men enlisted into the USCTs did so in southern rebel states) and it is very clear that the demographics of the rebellion were even weaker than they appear, based solely on population.
As an aside, there were loyal "white" USV units raised in every southern state but South Carolina, and USCT units in every state (including SC); there were no "rebel" units raised north of the border states.
Best,