There's no way to say what I want to say without sounding sarcastic, so I'm just going to say I'm glad this thread attracted Nytram's eye.
What do you think Johnston might have tried if he hadn't taken that wound?
Johnston seems to me like he'd be reluctant to be as fully "the best way to defend Richmond is to scare the pants off of McClellan", but if he has an army roughly equivalent to what Lee had int he Seven Days that would be close to numerical parity - allowing for how accurate Confederate intelligence is, but it's better than he had in other times and places.
Well, according to Craig Symonds, he wanted to continue the battle the next day, but there's little hope, I think, of much success at Seven Pines after the botched attack of the first day considering Sumner had managed to bring troops in support of Keyes and Hintzelman was nearby. I expect, therefore, that the battle would likely not continue beyond the second day, even it was handled differently from OTL when G.W. Smith oversaw events.
McClellan's reaction would be unchanged regardless of Johnston not being wounded and he would move most of his army south of the Chickahominy and look to bring his seige weapons up. Johnston would, as Lee did, see the opportunity of the isolated Federal Corps on the North side of the river and look to strike that, but he would have to reorganize his army before he could do so because he would have to integrate Theophilus Holmes and Roswell Ripley's forces as well as replace wounded or killed officers.
JEB Stuart's ride around McClellan likely doesn't happen, at least on the same scale, as Johnston is unlikely to be willing to release his cavalry to conduct reconnaissance in force in that manner. He would rely on Stuart to conduct some reconnaissance, and Stuart may decided to attempt such a ride regardless, but I dont think Johnston himself would give permission to ride off with a large force of cavalry and would prefer them to remain close to the Army.
The actions in the Valley, we can assume, will not be effected by Johnston remaining in command of the ANV because Jackson was operating independently of him at that time anyway, and one assumes that Lee - operating as effectively the theater commander in Virginia - would see McClellan as the immediate threat after the Valley Campaign had concluded even if he wasn't commanding the ANV and would call Jackson's forces to Richmond to act against it.
This would give Johnston command of the largest Confederate Army ever assembled. He would not wait for the Federal siege guns to come up, because he knows the Confederates have no answer to that, and he would not give up Richmond because he himself recognized that the ANV lost the means to sustain itself if Richmond fell.
He is compelled to attack at this moment and he would definitely attempt to hit Fitz John Porter's Corps at Mechanicsville. Much there depends on the arrival of Jackson and whether he performs well - he didn't in OTL - but, beyond that Johnston knows he has to attack and drive McClellan away, there's no other option at this time for the commander of the Confederate forces, he certainly not going to be as aggressive as Lee in doing so but I consider McClellan losing his nerve and pulling out just as likely an outcome.