Lockpick is a totally broken skill, I've been told. You're better off unlearning it and sinking the points into something else. (Too bad, it was one of my favorites in DD. Such a time saver!)
A lot of the skills do what they sound like. I'm probably not too much help there, as my characters are both warriors (a pair of brutes, really) and I haven't bothered learning any of the mage skills.
When you find alchemy plants, you can right-click on them in your inventory. If the right character (the one with the alchemy skill) has them, and has enough of the right combinations to make anything, there should be options that come up for potions you can make. This is another one that I decided wasn't worth spending points on, so I unlearned it two acts ago and I'm having trouble remembering how it worked.
I wouldn't worry about trading or embellish equipment. Loot and gold are plentiful, and the only time you might ever be low on cash is if you unlearn a really expensive skill.