Architecture can help with many of your problems. If you station your crossbow-dwarfs behind fortifications or (even better) on a platform above their enemies, they will be able to shoot at them, but unable to charge off to their doom. In the same way gateways, bridges or even choke points, can give you time to assemble your melee squads without a line of sight to their foes. Again, this way they won't charge off in penny packets, but will wait until their enemies come to them or you order the assault.
If you can build a courtyard or two in front of your entrance. Build a second floor, maybe with fortifications, maybe with an inner room with bolt stockpile. Won't cure everything, but will be a step in the right direction.
As for equipment instead of civvies skivvies, (m)ilitary--> (e)quip-->(r) This toggles between "Over clothing" and "Replace clothing." Probably will have to do for each squad. If you are using uniforms, (m)ilitary --> u(n)iform --> (r).
As for working levers. I know you need the mechanic labor to build and attach levers. If its a priority, pick 1 (or 2 or 3) dwarfs, enable mechanic labor and disable all others labors until the tasks are done.
Finally, as far as picking up corpses and goblinite, there could be a lot of things causing that--stockpile space for items, too many hauling tasks (we'll get there, just wait a bit!!), burrows, etc. One that might not be as obvious is if the items are forbidden (can you check?). If you go to Set (o)rders --> make sure you have Dwarfs (G)ather bodies on. And while you're there check out (F)orbid-->Claim (o)ther dead and Claim other death i(t)ems.
Last thought--armor and bolts can be tricky and glitchy. Make sure you have way more bolts, quivers and armor than you need. Make sure you are familiar with the military set up screen. I dress each dwarf individually--I don't know that I'd recommend that, but I always know what I need to forge more of. And check what your little beserkers are wearing from time to time--at least that'll spot problems before they lead to !!fun!!