Do the traps have to be reset? ^

No, but you need empty cages.

Getting rid of items as better quality versions become available. What is the strategy here? Is there a way to designate an item as "for future sale" directly?

You could forbid them, dump them close to the depot, and unforbid them when merchants are here.

Situational awareness of dwarves. Some of the dwarves in my melee squad were aimlessly wandering around the fort even after I've given a move order for that squad. What am I doing wrong?

What does <u> tell you, what are they doing? Picking up equipment/provisions can sometimes have higher priority than moving.

Cleaning up a mess. As my fort got older a lot of random corpses and other remains began appearing all over the pace (likely from hunting). After surviving my first siege the mess got even worse, with one unfortunate addition -corpses of my own dwarves. I've set up several stockpiles configured to store refuse and some garbage dumps, given orders to dump specific corpses but my dwarves just won't do it. Also my trap corridor got filled with all kinds of garbage (both items and corpses). Is there a way totally clear a specific area?

In order for stuff to be picked up and dumped, it must be unforbidden, that is claimed: d->b->c.

Crafting clothes for dwarves. I've read that after a while the clothes that my dwarves are wearing will start to wear off and will have to be replaced. Does this have to be done manually or will they craft them automatically if the resources are available?

Clothes won't be made automatically, but available clothes will be picked up automatically by a dwarf in need. You need a clothier and a source of cloth: a loom or merchants. Check out the wiki on textile industry for more info.