Using Tobacco as a Vegetable

Last spring we ordered the following tobacco seeds from you: Nicotiana tabacum, seeds and Nicotiana rustica, seeds
The seeds germinated beautifully and we now have wonderful mature plants in our garden.
I have several questions about the above plants, namely:
-Will frost destroy or degrade the niaciniminic acid in the plants?

I don't know the compound "niaciniminic acid". According to Jim Duke's "Handbook of Phytochemical Constituents of GRAS Herbs and Other Economic Plants" (CRC Press, Boca Raton, FL, 1992), there is no such compound in Nicotiana tabacum. Are you referring to nicotine, or possibly nicotinic acid? There are at least another nine compounds in tobacco beginning with "nico"; but none beginning with "niacin".
Nicotiana tabacum leaves contain 20,000-40,000 parts per million nicotine. N. rustica leaves also contain nicotine but Duke's database does not give the level. We assume though that N. rustica has higher levels of nicotine.

-We use the tobacco as a vegetable and therefore to preserve the leaves, do you recommend chopping and freezing, or dessicating and storing in airtight containers?

I must admit that I have not thought of tobacco as a "vegetable". It is a chewing herb and is, of course, a smoking herb, but I am not aware of anyone who is using it as a vegetable.
That said, the Plants for a Future database (https://www.ibiblio.org/pfaf/cgi-bin/arr_html?Nicotiana+tabacum) indicates that N. tabacum does indeed have edible uses; here is what it says:
"A protein can be extracted from the leaves. It is an odourless, tasteless white powder and can be added to cereal grains, vegetables, soft drinks and other foods. It can be whipped like egg whites, liquefied or gelled and can take on the flavour and texture of a variety of foods. It is 99.5% protein, contains no salt, fat or cholesterol. It is currently (1991) being tested as a low calorie substitute for mayonnaise and whipped cream."
There is nothing in the same database on using the leaves of N. tabacum as a vegetable, in either the fresh or dried state. And no edible uses are reported for N. rustica.

-Of the two varieties we acquired, which one contains the highest quantity of niaciniminic acid?

If you are referring to nicotine, we believe that N. rustica has more than N. tabacum.

-Do you recommend any recipes for preparing tobacco to eat?

Sorry, I don't have any recipes.