Great questions!
Yes, opportunities are specific adventuring leads with a payout. For example, a thief offers to pay the goblin double the value if they retrieve a specific item from a given location.
Goblins work at their job locations and make Scratch based on the Go to Work roll (page 17). It is intentionally a less-than-subsistance wage (unless you're a boss). In order to survive, you need to keep adventuring. Having to show up to work is mostly a bad thing.
If the goblins carry Scratch around with them, it usually takes up 1/2 Inventory per Scratch.
The latrines let you gain 1 Scratch if you make a Grim favor roll to possibly get Sick. The latrines aren't great. But upgrading them makes them useful; good fodder for a quest.
You ask about whether it's just these goblins being paid poorly in town or if the whole town is in rough shape: it's the latter. You can of course find your own tone here, but no one in Goblinville is living the high life.
Yeah, rumors are like opportunities but more vague.
If you still have questions, don't be shy. Often others show up with good guidance on getting started. :D