Every question of the OP is answered already in previous treads but once again:
1.Stealing/pickpocketing could help at the beginning.
"2. Are non-combat Personality abilities worth investing points in?" - Charisma is worthless,some items with charisma could be crafted, same for Barter- boots, amulet and trait- all you need even if you buy all the books from Teller of secrets. Leadership is a combat ability and gives you a big advantage and I'm afraid it'll be nerfed although this isn't mmo. Lucky charm is totaly useless.
3. Companions should be treated with the same care as the main characters, don't give them lousy talents just because they are called henchmen.
Your main character is usually better to learn craft/ blacksmithing because of a free point from a trait and you need to invest 2 hard points in crafting and 3- in BS, the rest is from a trait, bracers and belt. A mage is not very appropriate for such a role because he needs to invest in many other abilities.
Loremaster- 5 points needed- 1 from trait, 3 from an amulet and rings and 1 hard point to invest.
Telekinesis- I never bother with this one.
"4. Are any of the "Nasty Deeds" abilities worth taking?"- Yes and only sneaking, if you want guerilla.
Lockpicking and pickpocketing are absolutely worthless.
"5. What are the advantages/disadvantages of stealth vs. invisibility? " Sneak synergies with Guerilla talent, can be braked. Invisibility is almost guaranteed stealth and you can use a scroll for it, I found it useful for stealing and some special moments but you don't want spoilers.
"Obviously, if I want to cover non-combat abilities (e.g., leadership, loremaster, lockpicking, sneaking) with both that's going to leave me pretty thin on the crucial combat abilities. This is why it'd be greatly helpful to learn if they are useful or not and/or whether they can easily be covered by companions/henchman and so are wasted on the two main characters."- Only lockpicking is truly non combat ability. Loremaster helps in combat to see the weaknesses of the enemies especially for the first play through. And again: everything wasted on companions is wasted for the whole party so treat them equally. Because of the free points from traits, only the main two characters can have traits, it's usually better to have some abilities on them like barter, loremaster, leadership, craft...
