1. Left clicking and holding will have the characters follow your mouse cursor, keeping the camera centred on the lead character. It sounds like sometimes you are either holding the mouse button down briefly, or the game thinks you are.
4. Walk in Shadows works for me (tried it on a rouge and shadowblade). When the (placeholder) yellow horn blowing effect is triggered your character should almost completely disappear, except for some subtle blue highlights on the character/equipment outlines.
Does the combat log say that '<CharacterName> is Invisible' when you use the skill?
Did you try verifying local files for the game?
How about changing your graphics settings / texture quality?
1. That is on the to-do list. The person who joins takes over the non-lead main character, so you can already determine that.
9. There are a couple people in town you can recruit, as well as combat in town. The safest gate to leave through is the one that isn't closed, in the north west. Before that, though, there is lots of stuff to do and loot in town.
There is a tunnel under Cyseal with some combat appropriate for level 2 or 3; the graveyard entrance would be the easiest one to find.