To answer your first question: the North grew its own food just fine. The agrarian South vs. industrial North narrative is technically true, but taken unadorned ends up misleading. Compared to the South, the North has much more industry to boast. The South isn't devoid of manufacturing, but its investment in that sector is minuscule compared to the North's. That's the technically true part.

The thing of it is that the North is not a great, sprawling industrial heartland either. It has more industry, but the section as a whole is still primarily agricultural. This is missed in part because students of the nineteenth century have often focused on the more developed East (in particular Massachusetts) at the expense of even fairly nearby agricultural hinterlands. The Midwest spent most of the antebellum era, from the time of white occupation onward, engaged in farming. (Mostly family-scale and oriented around village-sized communities, though they are plugged into major cities and become more so as time goes on.) Ideally, though not always, they did so with an eye to profit as much as for sustenance. Some of that food went east, but a fair portion went down the Ohio and Mississippi to feed the Cotton Kingdom. There was an agricultural powerhouse down there which produced vast fortunes, but you couldn't eat cotton. Enslavers will tell one another that they ought to change it up, just as they ought to quit their dependency on Yankee credit and Yankee manufactures. They never quite get around to it, though there are occasional, brief boycotts.

So what really happens when war breaks out and the national trade network is compromised is that the CSA scrambles to convince planters to grow more food and less cotton. Instead they got shortages and some unrest, though neither is something I've studied in any detail.

That leaves your follow-ups. The loyal slave states did have food-producing ability, but I'm not aware of the North ever suffering a critical food shortage during the war. They were not sufficient to feed the textile mills, as cotton was not a viable crop that far north. The Border South's major crops were more on the lines of hemp, tobacco, and wheat. Hence when the US army appeared to be in control of an area, merchants were quick to follow to try to restart the cotton flow.

Sources

Huston's Calculating the Value of the Union discusses the nature and scale of the Northern economy.

Johnson's River of Dark Dreams lays out the whole trade network that the Lower South operated within, from the slaves' food (not enough) and clothes (low quality) to discussions of manure and the steamboats that fed the section.