So, I don't think the battle as portrayed, is really feasible. For starters, unless you're changing the weather completely, the Confederate army would be bogged down in wet and soggy conditions by the rain which historically held up the armies march on the 4th which delayed things for 48 hours. That means that, rather than a swift and well coordinated assault, you probably end up with a literal slogging match through over muddy ground in driving rain. At best Johnston must delay the attack to the 5th of April.
Secondly, while attacking with Johnston's original plan would probably have been far more successful in driving the Union forces back like Johnston planned, the idea that they could be driven to destruction against Snake or Owl creeks is just unsustainable. This is largely because, like OTL, to keep extending his line into the attack to dig at the Union flanks, Breckinridge's men would need to be deployed. Johnston just didn't have enough men to spare to keep them in reserve. The problem is also that the Confederates will increasingly be driving the Union into better defensive positions as the attack goes on, whether at Shiloh Branch, Tillman Creek and Dill Branch, or the final OTL defensive position at the Hamburg-Savannah road and Pittsburgh Landing.
Thirdly, Nelson's division would be well in time to arrive to shore up the Union defenses by the 5th. That means that fresh reinforcements could well be fed into the Union line in time to spoil any renewed assault the morning of the 6th.
Finally, there's no way Grant can be killed by Forrest in this scenario. That was not a battlefield favorable to cavalry.
That being said, while I do very much disagree with the narrative as described...it's actually set up a scenario which would prompt a Confederate victory for all intents and purposes. The Army of the Tennessee would lost probably double, if not triple, the manpower captured, killed and wounded. Sherman's command would arguably fall apart, Wallace's laborious arrival of OTL might, instead of finding the line instead blunder into the Confederate flank. The troops at the Hornets Nest would surrender as OTL, but probably with a lot more of their division. By the time more of Buell's men can form into line they are probably simply supporting Grant's withdrawal back across the river after having held their position against a harsh but ultimately futile, Confederate attack on the 6th.
However, that's still a huge blow. I'm willing to bed that the Confederate casualties are moderately higher, but not by much (maybe just shy of 12,000). Union casualties though will undoubtedly be higher, probably in terms of men captured or killed if Sherman's death causes his division to route. Probably putting the Union casualties at closer to 2,000 killed, 9,000 wounded and the better part of 4,000 captured (many of these wounded or surrendered), so that's still upwards of 17-18,000 casualties inflicted on the Union forces and it has probably compelled their withdrawal from the field.
With Sherman dead, Grant is also probably ultimately shunted aside by Halleck. This worrying turn of events though, would probably not allow Johnston to march north (he now has wounded and sick and his army is going to be out of rations, the rations that they ate pre-6th would have helped sustain an attack on the 6th) but it does probably cause Buell/Halleck to pause and reorganize. This is a godsend for the Confederates who would have time to breathe and possibly maneuver to the north.
My bet is Buell/Halleck would fall back behind Duck River, while Johnston, after his army has a breather moves to pursue after being joined by the Army of the West. How long that stalemate lasts is an open question, and is probably dependent on if Johnston can spoof Buell and Halleck. That being said, he's unlikely to be willing to engage the Union army which has probably seen some reinforcements from the East, will still be close to double his own size, and one which controls the riverine means of communication. Sending such a big shock all the way back to Washington, and buoying Confederate spirits in the West, will be a huge boon either way. Might even be more successful in holding more of Tennessee and pursuing a much more aggressive campaign in Kentucky.