The lower hull almost never presents a target except at the closest of ranges due to terrain and the angle of incoming fire. And your data on the KwK 39 is only correct for the PzGranPatr 39. Also note that the Germans tested their rounds at a 30 degree angle to the armor plate, and used German armor plate rather than cast steel.
From German field tests:
5cm Panzerwagenkanone 39 L/60
Fire rate: 15 per minute
Muzzle velocity:
5cm Panzergranate 39 685 m/s
5cm Panzergranate 40 1180 m/s
5cm Panzergranate 40/1 1130 m/s
5cm Sprenggranate 39 550 m/s
Penetration at 30°:
PzGranPatr 40 / PzGranPatr 40/1 / PzGranPatr 39
100m: 130mm / 116mm / 69mm
500m: 72mm / 76mm / 59mm
1,000m: --- / --- / 47mm
1,500m: --- / --- / 37mm
(Image removed from quote.)
All the penetration data I listed, even for the US 75mm M3, was against 30 degree RHA plate. The numbers are comparable.
I did not use the PzGr.40 because it is an APCR round. I personally never compare the specialty ammunition, especially on German tanks, because raw material shortages limited their use. Generally, if available at all, they had only a few rounds per tank, unlike the more standard steel shot-type rounds.
For example,
this site shows the raw production values of the ammunition, and that the PzGr.40 was only 14% of all 50mm AT rounds produced in 1942 (and that was the peak). However, it does not give information as to when and where that ammunition was deployed. I would imagine most of that specialty ammunition would have gone to the East Front where the Germans were encountering more and heavier armor, but I am "imagining" and can't find any information either way. If you have a source that gives any insight, I would be interested in reading it.
But using the 14% figure, and assuming a 50/50 load out of AP to HE, the IIIs in the desert would have at best 6 or 7 rounds per tank. If you want to base your gun comparisons on first round only, assuming that round will hit, etc. etc., that is your prerogative, but I prefer to use a common round approach. (Suppose I could have used solid shot for the Kwk 39 as well, since it was more common thatn the PzGr.39, which would have decreased the penetration by a couple mm's, but I was feeling generous.) If you want me to say the German gun was better 14% of the time, but the US gun was better 86% of the time -- well there, I've said it.
So, no, you still haven't convinced me the Sherman was inferior to the IIIs it faced.
Slightly off topic, it is interesting, though, that the "crummy" design of early German armor lent itself to being upgraded so readily, and therefore remained relevant and useful well past when you would otherwise expect.