Forrest as an army commander wouldn't be that far-fetched. Quite a few military historians have noted he had real talent, and that his younger brother Jeffrey and son Willie also seemed to have the same remarkable abilities. I don't think Forrest would have sought such a command if he could have - like many military geniuses, he knew where he belonged and that was in the cavalry.
His background as a slave trader was something that, maybe not so surprisingly, gave him certain military-useful training. He knew how to get men who didn't like him to do as he wished, knew the right combination of carrot and stick, and the psychology. He also knew how to move large bodies of men quickly and keep them fed and supplied. His lack of education did hold him back but it proved to be surmountable - Forrest was very adept at getting people in his staff who could fill in where he was deficient. His clerk was George Cable, very good with words and his chief staffer was Major Armstrong, who usually drew up the battle reports and official letters. Forrest dictated these - they are mostly his words - but the spelling and grammar was fixed. He would have whatever he had dictated read back to him 'to get the pitch' - he didn't read them over himself because his slow reading would give away his poor education. Semi-literate folks have little tricks to hide the problem and that was one of Forrest's.
Forrest could be a bully and could be more than some difficult, especially if he was dealing with someone he felt was inept. He positively hated following stupid orders, like Wheeler's order to re-take Dover. That went poorly indeed. His command of a corps at Chickamauga went well, and he even covered for some slip-shod commanders - like Ector. Ector needed somebody to plug a hole and mind a battery, which Forrest did, but Ector was fretful, kept sending messengers to ask about it. "Tell him I am here," Forrest said to the first, second and third guy. The fourth guy got the battle field roar: "Tell him I am by God HERE!!!" He sure was - he was wearing a tan linen duster and when he took it off, there were fourteen bullet holes in it. But he did well.
Tupelo - Forrest made a rare blunder that probably cost the fight. It wasn't that he was handling his corps badly but that he did not have his heart in the fight. He had just seen the Kentuckians get chewed up and spit out, and believed if he sent in more men they would meet the same fate. He wouldn't send any trooper where he wouldn't go himself, and that was one of those places he wouldn't go. Hindsight indicates that had he sent in those extra troops, the outcome might well have been a win.
Overall, I'd say Forrest was right where he was supposed to be. I do think he could have handled an army but the bureaucracy that went with high command would be intolerable to him. Up to a point, but beyond that he would have wished he was back in the saddle again! This is where military training would have solved the problem mostly - he would have learned to suffer fools. Not gladly, but suffer them. Soldiers say yes, sir a lot! He was certainly more used to hearing that than saying it.
Bringing the