"One more point, Civil War cavalry did fight from horseback, or at least Union cavalry did."
.....and as you mention until 1863, the confederate cavalry rode circles around them. Brandy Station, June 9 1863, is the first time man for man that the union cavalry held it's own against their southern counterparts.
"they learned the trade the hard way and were highly effective as cavalry by war's end"
This statment helps make my above point
Read about John Buford's fight with elements of Gen. Heath's division at Gettysburg. That was standard tactics. He fought dismounted and held off a much larger force of confederate infantry for a few hours. I say again, there were very few LARGE SCALE mounted cavalry battles.
Also, on the other side of the coin read about union Brig. Gen. Elon J. Farnsworth's death at Gettysburg. He died leading a failed mounted attack against rebel infantry from Hood's division..
http://en.wikipedia.org/wiki/Elon_J._Farnsworth
Mounted attacks seldom worked against infantry or dismounted cavalry, unless surprise was achieved (rear/flank attack) or the infantry was beaten and demoralized and the cavalry was thrown at them to "hasten" their retreat or catch a few prisoners. The main use for mounted troops in the civil war, was scouting, screening, and skirmishing. They were no longer the "shock troops" that they had been in previous wars. The rifle musket saw to that.
"but his muzzle loading shotguns would not have been very effective against Spencers, and he knew it."
That's why he used them against sabers. Morgan's men adopted shotguns to replace only their sabers. They still had rifles/carbines and revolvers.
"Mosby favored the ambush, where his men would have been effective almost regardless of the weapons carried by the Union forces since they never got a chance to use them."
If you read the link I posted, you will notice that at Miskel's Farm it was Mosby that was surprised. He was caught and outnumbered more than 2/1. The "yankees" had the upper hand. He won because Capt. Flint decided to order a saber charge. Mosby's men fought back with revolvers. Flint was one of the first men to die (with 6 bullets in him I might add).