‘Think how much the western theatre would have changed if Johnston would not have been killed at Shiloh. Most battle experts believe without that event the results would not have been a stalemate. i.e. dominoeing to Vicksburg, etc. etc.’
I think you can make the argument Albert Sidney Johnston was one of the top two or three flag rank officers in the entire CSA. Would that have carried the day against Buell and Wallace’s additional troops?
The math says no, but the CW was a war where numbers didn’t matter in so many battles you can’t say for sure. The South was almost always outnumbered.
I've seen A. S. Johnston's ability disputed, I don't know with how much truth and fairness. But had he not been shot, his critical task was to carry the last line of defense and drive Grant's troops off Pittsburg Landing by the close of daylight, which the Confederates failed to do in part because of the delay created by Prentiss, then the delay caused by the fighting for the Hornet's Nest, and also the further delay caused by the command reorganization entailed in Johnston's death.
Johnston's and Beauregard's troops just had a long, long way to go to carry their objective. I've often thought that if they could have stolen a march by advancing through the woods by the river bluff and thus stayed out of sight while they flanked Prentiss to the east, they might not have had so far to go......but I'm sure there are arguments why that would have been a very bad thing to do -- it would have exposed them to the gunboats on the river, for one thing, and to flanking for another.
I don't think you can make that case at all. Johnston will remain one of the great unknowns of the Civil War since he was killed midway through his first day in battle. In July 1861 you could say that Joe Johnston and P.T.G. Beauregard were the team to beat, but history has proven otherwise. When he was first appointed McClellan was supposed to be the boy genius of the battlefield and again, history proved otherwise. With the other Johnston we will just never know.