I need the throat hit of higher nic. I dunno about turning it up (especially with a sweeter juice, might loose a lot of flavor; or even burn it) I like a warm, but not hot hit. I keep mine at about 18W with my (sugar cookie) juice. Nice hit, good flavor.
If you wanna turn it up, just go slow, every few hits up a watt or so.
I've not been nic sick for years. (went from the exact same mg in a nautilus tank at 1.6ohm to the subtank. Just kept vaping like I always did)
I use about 6ml of juice a day in my rig.
I dunno about the old dark juice, but I have several bottles that are over 2 years old that I enjoy on occasion. a couple of them look almost black. Give it a try, can't but not like it.
YMMV, but if it's too old it won't make you sick or anything, (likely it will have lost a couple mg of nic strength) it'll just taste bad.