Prepared meals don't have their own category? or they do not fall into a category?
Both
Prepared meals will only directly be found in the 'all' category, when searching for 'roast', 'biscuit' or 'stew'; and that will only catch those outside of containers.
If your prepared meals are in barrels or pots, you can find them in the 'barrel' and 'tool' categories. Just look at those categories, ask for 'prepared' (because they're all 'prepared food barrel/pot') or 'sort by value' - they'll be at the top of the list normally.
The trade interface is somewhat broken in that there are some items that have no purpose but export which can't be properly selected in the 'move to depot' interface. Most annoying are goblets and totems for me. They'll be stuffed into finished goods bins but don't have a sub-category where they can be found (unlike crafts, and instruments and the like).
I know I have seen the crafts category in the past, But I cannot say which race was at the trade depot. So I don't know if it is race specific or if it doesn't show if you don't have any.
You only get the categories displayed if you have any items that actually fit them. So if there wasn't a 'craft' item in the list, you either had no crafts (the sort of junk created when ordering, say, 'rock crafts' - rings, bracelets, figurines etc.) or the bins containing them were all in transit; or they were all hidden because of export bans.
I generally separate my finished goods piles - one is for export junk - crafts, goblets, totems; another for clothes. All other finished goods normally stay off the stockpiles - jugs, nest boxes, hives and ropes need to be available for building/constructing/pressing all the time, having them crammed into bins and carted all over the fort is massively counterproductive. And quivers and waterskins aren't much use in bins, either.
Can I cook meals from multiple items of the same kind?
Will dwarfs consider a cooked meal to be a different kind of food from its base components?
Making meals out of multiples is entirely possible (yummy *dog tallow roasts* containing dog tallow, dog tallow, dog tallow and dog tallow are common); can't help you on the second question, i've never been in the situation to find out.
You could allow dwarven wine for cooking and hope plump helmet + wine biscuits count for variety. You could also cook some of the excess plump helmet spawn, but you'd have to be careful not to accidentally cook yourself out of seeds; mixing seeds and booze in cooking doesn't seem to work properly, dwarfs prioritise seeds when cooking.