Logistics

[loki100]

Just to reinforce the above, not only was Stalin utterly culpable for the orders to SW Front at Kiev but he opened the way to allowing the Germans to strike south from around Smolensk. In effect, what was known as the 'Bryansk Front' (2 battered armies) could and should have had 5 Army (holding to its south) subordinated and thus reinforced.

Soviet records describe this sector by mid-August as the 'lid' that was keeping the Germans blocked, Stalin weakened it, opening the door to the Ukriane and damning SW Front to destruction. Even then, if Kirponos had been given freedom to move, the encirclement would not have happened.

[Seminole]

They marched south against sketchy opposition and even then had a rather rough time of it.

But this 'sketchy opposition' would have presented a genuine threat to Bock's flank? It seems you want to have it both ways.
You say they couldn't have marched 240 miles, yet they went 300, then they pivoted back in less than two weeks and launched another huge operation and pocketed a force larger than what would have faced them if they had gone east originally.

Guderian's PG was in terrible shape at the end of it.

And yet he still made hash of what the Russians scraped together between Smolensk and Moscow from August to October in less than two weeks. And accomplished this after making the march south (over a distance you assert was impossible), then the march back northeast and participating in intervening weeks of combat.
I see no reason to believe they would have performed worse than the first 10 days of Typhoon if they launched it in mid-August instead of October. The Reds before them would be greener, less equipped, and PG2 would have avoided the casualties and wear and tear of the diversion south.

It worked for one reason only: Stalin's insistence that Kiev must be held.

And yet we're to believe that 5th Army would somehow disengage and attack the German 2nd Army around Gomel-Mogilev (whilst being driven east by the German 6th Army) and outflanked by the PG1?

AGS was stalling prior to the pocket and PG 1 was split into two groups, one of which was trying to hold a bridgehead near Dnepropetrovsk (that was being contained handily) while the other was seeking to cross the Dnepr to meet up with Guderian. Without Guderian it is difficult to see how either of these efforts would go anywhere let alone achieve decisive results.

I dispute that AGS stalled. They pushed north around Kiev to pressure 5th Army, and von Rundstedt ignored Hitler's panic so he could press farther southeast and bag everything he could west of the Dnepr in the Uman pocket.
Furthermore, PG1 linked up with PG2 over 100 miles east of Kiev. They didn't sit on the Dnepr waiting for Guderian to show up. AGS proved more than a match for Southwestern and Southern Front. I think the argument that these Fronts could have managed to divert strength to assault 2nd Army on Bock's flank without collapsing even faster is unsupported.

[Flaviusx]

The sketchy opposition I am referring to came from Bryansk Front. Which mostly didn't amount to much. The main problems Guderian ran into where supply, terrain, and the need for additional forces for flank security as he wound his way (not very quickly) south. He basically was marching through a major gap in Soviet lines. And even doing this his panzer group went to pieces. Model's division was down to 10 runners by the end of it. The others weren't a whole lot better off.

If this same panzergroup had marched due east against actual opposition immediately following the Smolensk battles, I think it would have not done so amazingly. Bear in mind that Guderian's initial success in the historical Typhoon was based, again, on marching through a yawning gap in Soviet lines. Bryansk Front's flank in the south was hanging mid airas a result of the big Kiev pocket.

So far as AGS goes, Stahel proved to my satisfaction that the lower Dnepr bridgehead was a near fiasco and ongoing drain on supplies and reinforcements. It wasn't doing anything or going anywhere. Halder insisted on holding it mostly due to pride, near as I can tell. It should have been evacuated. It pretty much was the AGS version of the Yelnya salient further north in AGC. The one further north was more promising and could operate in conjunction with Guderian. These two bridgeheads offered no mutual support, btw.