I don't remember much but:
1) Yeah, you should repair them before getting to 0 since it'll save your repair powder consumption (it costs more to repair a broken weapon).
2) IIRC there's a part of the status screen shows how much you can get out of a material. Crystals always give out 3 points, Rocks and miscellaneous stuff gives out 2 points, and some other stuff gives only 1 point (and some that gives more than 3 points). Try to only syntesize items that'll give you 3 points (Crystals) to get most out of levelling your weapons although not necessary.
Also level up your weapon to max (no pun intended) befor synthesizing them into a newer weapon. A quick way to get stronger weapons is to use the MOST basic wrench you can get and pick up EXP points on later/higher levels (kill enemies with a strong weapon like guns, but pick up the EXP while equipping the basic wrench you have).
3) I'll have to check this out later since I forgot some stuff about the game. Try checking the tutorial.
4) I don't think so. You'll have to manually memorize it. Also a WARNING, Scoops are achievements of taking pictures of rare stuff (mostly bosses) which nets you valuable items if you get the scoop. So don't kill any boss before taking their picture!! I missed scoops of the first and second boss, so I can never get them again. While you're still early-game, restart if you missed them (I realized only until 25% through the game ;_;). I think I might restart DC2 instead of continuing my save file...
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...then blame it to the cabbages