Disclaimer: Typing at the end of 9.5 hour shift.
To answer your questions clearly the issues are this:
Cons have an ABSOLUTELY abysmal base (1.09 model is just awful when the main base is 1) and only beat grens/volks by simple virtue of outnumbering them and getting up close. For years they were a meme unit because their late game was just so weak despite successive buffs. 7 man went a very long way and they're fine now. Their base damage is pretty low, esp at far and the vet buffs go a long way towards making them a reasonable unit. This graph is of the base damage values and it's clear that in a distance cover shootout they'll get crushed hard. Volks have 31-70% more damage if 1:1 squad comparison. Obviously 6 men but you'll lose a guy fast with base model size. Vet II+ obers and pzgrens will still walk over even vet III 7 men cons.
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Penals start off extremely well but they scale not as well. (Used to be even worse, imagine running around with 1 model size inf vs pzgrens and obers!!!) That price for their fantastic base has to come somewhere and scaling is one of them, as is the cost of the unit. Considering they start as 6 men and have this firepower below? Strong scaling would make them ridiculously OP.
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Of key importance is the fact that volks and fusilier do not have the same Kar 98. That's up to 25.7% more damage at close range. Yes the upgrade is more powerful for fusiliers than volks but it also costs more and the 6th man is not free. It is true that fusiliers scale very hard but that says more about how weak volks are late game than anything else.
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It's also not true that 40% accuracy is unusual for mainlines
Grenadier: +40%
Osttruppen: +40%
Conscript: Already stated as 40%
Riflemen: 30%
Fusilier: 40%
Panzergrenadier: +16.7% then +20%, making +40% overall
Volksgrenadier: +30%
Infantry Sections have weaker scaling (25% accuracy etc.) in great part imho because of how powerful their base is.
Penal: Last man stat bundle, +30% and then +20%