IG economics are super interesting, but my experience is that trying to enforce any of the reasonable logical rules on it IG *does not work well.* For example, in the real world the majority of our income goes to housing. IG, for the most part unless you're doing something special, you will never have to pay coin for housing. In fact, the idea of having to keep track of and pay IG 'rent' isn't appealing to most players. In addition, IG people want to get something in return for their money- paying out for between game 'rent' would feel like a loss since you aren't experiencing the benefits of that payout.
I'd say that the majority of coin that crosses hands between players is for ritual scrolls and food (and other amenities). The economy guides in the rulebook break down when confronted with real world goods, in our chapter. I believe a horse in the rulebook costs 3 gold, but some of our players sell energy drinks for a full gold each. Woah! A horse for 3 Rockstars?? Incredible!
To go one by one:
Trap crafting is generally a wasted stat (SF has custom lock crafting rules to help with this I think?): I rarely see traps set up by PC's in game here, but I know some of them do. I think the lock crafting rules you're referring to are that we use knotted ropes to represent locks in our chapter because of OOG laws about owning and carrying lockpicking sets, which other states don't have (The Denver chapter uses actual (though simple) locks which have to actually be picked).
Potion crafting is generally low demand because of casters being everywhere: Definitely not true! There is almost always demand for reasonably priced potions and alchemy, *especially* if you have a clear and easy to understand price list!
Armor crafting ends up being made obsolete by arcane armor: Kinda! Not everyone in our chapter has an arcane armor, and a number of characters have IG reasons to refuse to wear arcane armor! However, I very, very rarely see armor shattered or destroyed, so there's rarely need to buy new sets. Blacksmithing in general is a reasonably helpful skill, since arrows are constantly needed.
Arrows are one of the only consistently high demand craftable items: When you think about it in terms of ratios, an archer can go through 50 arrows in a fight easily. I've never seen a fight where one person goes through 50 potions. So in terms of sheer volume, you are correct. There is sincerely use for potionmakers though.
End game players have little to spend their vast wealth on: Depends on the player, but this can certainly be true for long chunks of time- and then there will be something we'll need several hundred gold for and then they all pony up! I'm not endgame even a little bit, but my experience with watching wealthy characters is that instead of consistent small transactions (a few gold here and there) there's significantly larger (hundreds of gold) transactions, but much more irregularly paced and only every once in a while. Our game also has (especially Maelstrom!) unusual or atypical things that can eat a lot of gold really fast.
I studied economics and while IG economies make sense when combined with the OOG realities if they are only taken from an IG perspective they make zero actual sense (which is why you'll never meet anyone with OCS economist- they've all gone insane years ago from the Lovecraftian mystery!)
If your goal is to make piles of IG dough, selling OOG goods (food, drinks, services, even costuming bits) is the best way to do it. Doing so will take up a lot of your time, though, and presumably a certain amount of OOG money. Personally, I've found I accumulate enough wealth to continue just by playing the game and participating in the storyline- which is what I come to the game for!
(or you can get an IG sugar momma, cough cough)
What sort of income per game do people tend to bring in? Obviously this is going to vary hugely by level, but I'm trying to get a sense for the cash value of player hours.
A lot of locations seem have an NPC merchant's guild that sells things. How is that handled in SF?
1. Definitely depends on the player, their character's goals, how many OCS levels they have, and their overall level. My character doesn't like fussing with money, so (with heavy fluctuations) I make a profit (so after I've bought things I want to buy at games) of about one gold per game I attend (but some individual games I lose a lot of coin, some games I gain a lot of coin).
2. We have two unique campaigns, Maelstrom and Enerret. Each location has IG roleplayed economy methods based on the IG location. Depending on our location in Enerret, there's usually some form of a merchant's guild or at least a market. Maelstrom is more complicated since it's post-apoc, but in the long run that's an IG roleplay question rather than an OOG logistical question for our chapter. 
