Hey there. Let me see if I can help with these. I'm a bit dazed at the moment though (I'm preparing for job interviews) so I apologize in advance for the gibberish I'm about to write.
RE: Spells and Weapons and Things
It does seem odd that a 0-level spell and a 3rd level spell (especially one as iconic as fireball) do the same damage. Similarly an unholy flaming +3 longsword seems like a more powerful weapon than, say, a table.
Let's take your example of an acid globe and a fireball. Both default in the offensive stance to providing 1d of damage plus their associated damage types. So in default mode an acid globe contributes 1d (acid, magic) while a fireball contributes 1d (fire, magic). This represents the minimum level of effort a magician can make; he's not planning, thinking, or even describing his actions. The player's description might sound like "I throw a fireball! Fireball! Fireball!" *insert player jumping up and down here*
The second option is to make a Spellcraft skill check, representing the character's effort to control not just the power but the placement and impact of the magical effort. In this case the spell level plus its function (damage dealer or not) impacts the maximum number of damage dice the spell can possibly inflict. So....
Acid Globe - Max 1d (0 level spells have no maximum damage so we default it to 1) (acid, magic)
Fireball - Max 10d (3rd level area of effect spells deal a maximum damage of 10d by the rules in Core Rulebook II) (fire, magic)
Now we begin to see a difference. It doesn't matter how well the player rolls on his Spellcraft check with acid globe - he will ALWAYS do a maximum of 1d. Fireball, on the other hand has a hefty 10d cap; more than enough to accommodate most Spellcraft checks.
So, why would you use acid globe at all? It adds acid to a threshold, something that you will need on occasion.
RE: Position
This is a tough one to get a handle on. I'll give you the simple mechanical answer then delve a bit into the more esoteric parts of the logic.
Mechanically position affects targeted damage and tags the character for conditions/events in the template. If a character in Ranged position directly targets a character in Melee position (or vice versa) he suffers a -1d penalty, mitigated by feats. Your position may also invoke a conditional or event effect (say, Event (characters in ranged position): +1d general damage each round).
More esoterically the position variable describes your relative placement with regards to the encounter. It takes the place of the entire map-variable in standard d20 combat without giving you even a quarter of the functions. The map describes flanking, movement, range, penalties, types of allowed attacks, environmental conditions; position just gives a SWAG as to where a character stands.
Honestly I don’t use the Position variable much in my own games. It’s included in the book because it helped smooth the translation between standard d20 combat and the NC rules. If it makes you uncomfortable just drop it.
RE: Actions
I'll first break this down mechanically then bore you to death with the reasoning.
For spells: It doesn't matter. Standard d20 controls spells though a combination of map, resource expenditure, and round-time. NC controls spells by skill check/resource expenditure. Since we ditched the action-oriented round structure for intent-based "pulse structure", the action rules went right out the window.
For fighters: A greater number of attacks (basically, more dice of damage output) comes with raised Base Attack Bonus. In NC the greater your BaB the more likely it becomes you will generate multiple dice of damage. Fighters also generate generic types and extra dice of damage though feats.
The theory: Stances are not actions. A stance (offensive, defensive, active, support, or passive) represents the character's intent in the scene. The variables he selects represent how he wants to go about realizing that intent. He then describes the effort and decides whether or not to make a check or use the defaults for minimal effect.
Since a stance is not an action, a round does not take six seconds. It represents a scene "pulse"; basically the space in which every character in a scene acts then pauses to reassess the situation. To see this structure in action fire up your favorite action movie. Take a look at the way the director paced the exciting bits. It usually comes out as activity - breath - activity - breath - activity - pause or end. That's the structure of NC combat rounds; we use them to break up the action into understandable chuncks rather than control action timing.
RE: Why run Narrative Combat?
Because you are the GM and you want to?
In the end the choice comes down to what you want for a specific situation. Do you want highly detailed tactical combat? At times, you absolutely do. Other times you want to dispense with minor battles quickly, make combat the spice rather than the meat of a scene, or do things involving radically different scales than the tightly focused skirmish level combat system sustains. In those cases NC steps in to give you another tool set; not a be all end all set, not a best thing since sliced bread set, but a working set designed to help you do it consistently and fast.
Shannon