I think the problem in thread arises from the difference from food to meals. I don't use masterwork, so the following is for vanilla farming.
You Do Not Need Meals for the dwarves to be fed. Their hunger is fully satisfied with a raw plump helmet.
If you are making roasts out of straight plump helmets, you are doing it wrong. This is something that should never happen. You do NOT get a seed from COOKING, but you DO get a seed from a dwarf eating a RAW PLUMP HELMET, as well as from using them for brewing. Therefore, wait until you have many different types of food (and an excess thereof) to make roasts in the kitchen.
So. take plump helmets off your cooking list. Have Chef Dwarfsalot cook something else. You can satisfy basic needs (both food and drink) with nothing but plump helmets. Your dwarves will eventually get bored of it and get a minor unhappy thought. This is solved by having multiple types of plants. You can trade for fancy food and drink, or trade for above-ground seeds (my preference is the latter).
Honestly, whenever I farm, I have one max size plot. This plot can last you up into the hundreds of dwarves with any non-dabbling farmers. But really, for an early fort, half that should do fine. Just make sure you have enough farmers (maybe 6 at your population? Probably excessive but its always good to have a surplus). 100 food is not enough food. If my food or drink drops below 500 I flip my wig. You want to make sure you have a surplus, especially since you are going to have alot more than 20 dwarves in the near future.
Oh, and remember your farmers don't need to be dedicated farmers. Set some masons or what-have-you to farm. They can pick up the slack when someone else is a'sleepin.
As far as "forbidding" a door, you can lock a door by going over it with [Q] and toggling it.