"I see it as misguided to proclaim that going veg will save our planet;"
It surely is - if you point me to where anyone made that claim that I'll sort him out for you It is likely a (more or less) essential step in the 'planet-saving' process though, that many more people go vegan and veggie. In reality there is more than one direct threat. I'm sure some of your points have validity if we blunder on regardless. What you say would imply that human population growth and intensive farming are the general problem. If these are not addressed then there might not be a genuine planet-saving solution.
I think there is a large gap in what you have said. You do not refer to the additional use of land, water and energy to grow crops for animal feed. A most inefficient and wasteful use of resources. Without factoring this in I don't see how you can say there would not be enough food for a given (or projected) population level if we switched away from the lets-call-it Macdonalds culture.
But there are also issues of degree. Not everybody would be vegan or veggie, and there would be a lot more land to use for crops. I would go along with you regarding the need for smaller scale, and the need to pay more direct attention to the crops. Lifestyle changes would ensue - a hard sell for those who would rather carry on until the damage is irreversibly done.
While your argument has relevance I am far from convinced it pours more than a warning level of cold water on the sustainability claims made for veganism. It means we need to do more than that.
Thanks for the exchange, I'll read any reply but might leave it there. Cheers