a) its not
b) yes
Dungeon gear is comparable to hq crafted gear, aetherial gear will have one other random stat
c) if you don't craft you cannot meld, would have to ask someone to do it for you, probably at a price. Melding gear is really only useful after lvl 50, in my opinion.
d) extra income
e) sure, but all DoH and DoL do tie in together, or you will spend some gil only doing one.
Chicken nuggets all day every day. You will spend gil, how much depends on what you hope to accomplish. I have all crafts and gatherers to 50, spent alot, but made way more than I spent selling stuff.