1: You cannot mod the eating rate, sadly, and I don't imagine that changing soon. But, it you burrow your dwarves and don't include the food stockpile, then dwarves will wait longer until leaving the burrow. This may be an essential part of an end-world fort.
2: You cannot mod the seed drop rate or the stack farming number - if it can be grown in a farm, then it will produce seeds when eaten, and will produce 1-5 plants per harvest without potash. But, you can mod in custom reactions with variable output. It can take 1 farmed plant, and produce however many seeds you want, or however many food items you want. In this way, I'm hoping to make no plants directly edible, or very few. Instead, you harvest a plant from the ground or from a farm, and you process it in a custom workshop, which may or may not yield edible material. Using this, farmed plants can sometimes produce seeds through processing, and identically named plants can be used for a random chance of poison. After all, if you take "raw berry" and process it into "berry" "berry" and "berry" how do you know which, if any, are properly purified or not? Or, which have been skillfully purified and provide an edible strength bonus? Or which have been terribly used and actually catch fire on the workshop floor? Custom reactions provide a TON of options, requiring an extra step or two but luckily there's already an [AUTOMATIC] tag for reactions.
3: I'm not sure if the new edible syndromes and curses will allow addiction, but I'm hoping so.
4: I'm planning on making golem enemies that drop rocks/ores on death (purifying an ore may or may not make it explode), but due to resource scarcity here, ground rocks will likely be a cookable ingredient. "This is a cat tallow roast. It is made with cat tallow, berry, berry, and ground limestone." Anything to fill the stomach, amirite? Most resources will have similar multi-use ideas in play, allowing for some questioning of "do I want to make a stone axe, or a meal?"