Why are you surprised Paul? I told you it was acting like a 13% protein flour
The last time this came up, I got a little bent out of shape that you were getting what was supposed to be a 14% flour that was acting a lot like 12.6. I would definitely keep shopping around (like I said, talk to bakeries), but I guess a 13ish flour isn't the end of the world. You're going to want to give it a little more kneading than a 14, but you really can't push it too hard or it will start failing on you.
I did little research and came up with a very informative article on farinograph testing (open the PDF so you can blow up the graphs):http://wheatflourbook.org/Main.aspx?p=33
According to this, under 'standardized' mixing conditions (whatever that is), weak flours peak in consistency around the 2 minute mark, while 'strong gluten' flour reaches it's maximum potential in around 10 minutes. Now, this is in the context of a mix, proof and bake scenario. With the additional kneading equivalent of a cold ferment, these numbers will shrink considerably. 12.6 is hardly what I'd call weak, but, at the same time, it will be less forgiving than 14.
This article basically confirms what my experience has shown me, namely, that, although, in theory, one can take a relatively weak flour and knead it longer to make it act more like a higher gluten flour, the potential for overkneading is so great that you really have to know what you're doing in order to pull it off. With 14% flour, you get a much larger target to hit when it comes to underkneading/overkneading.
Assuming you're going with a multi day cold ferment, with a mixer as powerful as yours, I'd say no more than about 5 minutes on the highest setting. You'll figure out the required kneading as you open the dough into skins. It will always be smooth due to the cold fermentation, but the elasticity and tackiness will vary. With an elevated hydration, minimal kneading and a quick bake time, you should have good extensibility/not too much elasticity, not too much tackiness and a tender open crumb in the finished product.
The biggest potential obstacle you're going to encounter is having one bag of flour being in the 12.6 realm while the next is 14. That's going to create some extra work as you'll need to do some testing with each bag to see how well it absorbs water. I don't think you need to make a whole batch of dough with each new bag, but I might measure a few grams in a cup add a set amount of water and see how easily it comes together. You could end up with a butterfly effect of sorts- a small shift in protein in a new bag might have large changes after cold fermentation, but I think if you're careful and aware of the issues, you might be able to work around it.
The convection part of your oven actually clears a lot up. Convection encourages even browning. I've seem some commercial pizza ovens that were convection, but I think most are not. You might want to play around with baking a few pizzas at home without the convection feature so you're prepared for the different results your commercial (most likely) non convection oven will give you.