I'm pretty sure that Rock Nut Paste and Press Cakes count as solid ingredients for cooking. I've had trouble getting the Oil to be used, so I'm not sure that using a screw press really doubles your food supply. What it DOES do is add even more micromanagement hell as you have to maintain an adequate supply of empty jugs to be filled with the rather worthless oil. I have only seen Oil used in cooking once, and it was in a Roast that contained 3 units of well minced Oil, and one unit of Yak Tallow (I haven't been able to keep my food supply 100% pure Rock Nut, since I had to do away with some animals).
I had the genius idea of also trying to have a wool-based clothing system as another part of this challenge, but found that I couldn't sheer and spin the wool, since my Farmer's workshops were constantly overloaded with "Process to bag" commands.
Although I believe it is possible to feed a fortress on naught but Rock Nut Paste, I do not recommend it due to the huge amount of micromanagement required at nearly every step of the process. Not bothering with Oil makes things easier, since you don't have to worry about Jugs, but you still need to queue up a bunch of "Grind to paste" jobs for your querns and face the constant battle of dumping leaves to free up your bags. If the workshops are located near the garbage chute, then emptying the bags goes very quickly, but this doesn't get around the fact that you're bombarded with job-cancel spam whenever you run out of bags, or your Planters Millers and Threshers are fighting over the same bag of Rock Nuts.
Rock Nut Paste as a food source is a pain in the ass because it requires your constant attention to do trivial and repetetive tasks which are difficult to automate in any reliable manner. It's do-able, but I think its best use is just as a material source for vegetarian soap. Rock Nut Oil is worth less than the containers that hold it. A no-quality stone jug with Rock Nut Oil in it is worth $15, and $10 when empty. An "Oil Exporting Economy" is a Jug-Exporting Economy.
I had dreams of feeding a fortress on massive quantities of Fried Nut-Cakes, but I think I'm going to give up on this in frustration. There's just too much busywork involved in the extraction and processing of Rock Nuts into foodstuff. (I had well over 2,000 units of leaves before I decided an atom-smasher was going to be a necessity. They just don't rot!) It turned into a non-stop drama of "not enough bags and jugs" which distracted me from getting a proper military going.