Okay I'll answer all the questions I can.
As of right now the level limit is Lv59 but in Dec Outspark is going to raise it to 79.
No there is no other way to get skill empowerment other than leveling, so you them wisely.
I would say pot production and magic decomposition are good ones to have. You can make alot of money with magic democomp.
You can make the items if you have learned the skill and have the items (learned the skill meaning you bought the scroll of what your making and right-click on it).
There is no possible way to exchange items between your characters without having to put the items in your storage and then going to the other character.
I dont know anything about any other 6th skill.
Hope this helps ^^
Edit:
Click here for more information on production.