During leveling, mix it up as needed to level the skill lines. Even if there are only a few key abilities in each tree that you want, pre-50 is the time to get them. Once you finish leveling your character, specialize as you see fit.
You can play around with the skill lines and always reset your skill points later. Having fun experimenting is half the fun no? Probably a good idea to have one of each skill on your bar while you level though so you can max out those lines.
Can go either way. You still gain experience in your morphed abilities so you can decide weather you want to be a something like a swiss army knife and do all things but master none of them, or focus on a select few and see them through.
up to you.
honestly with 320 potential skill points, you'll be able to master a line or two and have plenty to spare. As a sorc, even if you plan on utilizing dark magic the most (I do) I'd recommend picking up the storm calling powers. On some boss fights the CC abilities from dark magic don't do much good, so its nice to switch in the dps powers from storm calling.
I had the same question, and sadly, have only put into weapon skills (two handed weapon) and trade skills (blacksmithing) thus far. I play a templar, so will put one into the initial healing eventually as I will probably need it.
I am happy to see it is believed we can "reset" our skills at some point as I believe I will spend in the wrong place from time to time.