We can trivially feed 11 billion today. The farmland once used in America alone could do it (though that would be a bad approach for many reasons).
Your ideas about nutrition are way off. Calories are key to survival, and meat is not where you get calories, carbs are. Meat is a tasty luxury that requires more farmland per meal than eating vegetarian.
Fresh-water availability, as I already said, is only an issue in large cities that insist on drawing down their aquifers (well, and a few low-population areas with regular drought). Cities tap their aquifers only because it's cheap compared to proper sewage reprocessing. No magic technology required, just infrastructure spending. There are very few big cities that actually lack the surface water (e.g., Dubai), but they have desalination already. Wikipedia has some notes on the plants currently under construction and operating around the world. Again, it's not high-tech, as long as you're on the coast.
nd while economic development might wind up with individual families having fewer kids, that doesn't mean total population goes down
Native-born net population change is either negative or barely positive in every industrialized nation. Many places with high barriers to immigration are in population collapse right now (e.g., Japan). America is only growing due to immigration. It's a common pattern, well, researched and well understood. People have enough kids such that enough survive to help them in old age. Pre-industrialization, that's 10 or more. Post-industrialization that's 2-3, or fewer once a good retirement safety net is in place. There's a one-generation blip seen in most places during industrialization when people are still having 10 kids, but all of them survive to adulthood, so population explodes.
The news that population was expected to peak at 11 billion is at least 10 years old - not sure why it's a /. story, but we do like old news here.