I think it was the combination of bad terrain and Beauregard putting too much trust in Johnston to actually be a general, not a glorified colonel. Which admittedly was a sign of how inexperienced everybody was at the time (as Grant very obviously did not expect an attack. Even though nobody entrenched, the strategic surprise was there, his memoirs be damned in that case. The sign of his skill is in not going into panic mode and directing the battle the whole way through and rather mercilessly exploiting Confederate mistakes).
Yeah. At best Grant was caught wondering why his pickets were being driven in. At worst...
But as you said, he reacted effectively.
True, though I was thinking more about Beauregard in command in Virginia, where his skill and less bloody means of waging war would have been better than Johnston the master of retreating and writing memoirs and Lee the slightly milder John Bell Hood. I can't see Beauregard being so stupid as to launch a Malvern Hill and he did more with less than most CS generals did.
Agreed so far.
Against McClellan that combination might have actually been far deadlier than Lee's extremely risky strategy that in actual fact did produce a sequence of victories for Fitz-John Porter. A factor that tends to be overlooked.
Hard to say. Given McClellan's response to those attacks, it couldn't have fared much worse, and that's assuming Beauregard is even worse than Lee somehow.
The OP, of course, runs aground that McClellan was more abrasive and autocratic than Stanton, whose leadership can best be summed up as a J. Edgar Hoover of the 19th Century, minus the cross-dressing.
Possibly slightly more respectful of the president. Or at least less actively opposed to him.