There was a change to how contests were resolved part-way through development, and not everything related to that change was adequately amended.
Overall: when resolving a contest, any external factors which would make the attacker’s job harder or the defender’s job easier (i.e., any difficulty increases for the acting character, or difficulty reductions for the reacting character) count as an extra success for the reactive character.
Similarly, any external factors which would make the active character’s action easier (or the defender’s job harder) reduce the number of successes the reactive character scores.
So, you’re making a ranged attack against an aware opponent in cover (i.e., they can defend themselves), that’s a normal contest. If they’re distant rather than nearby, or if harder to see because of a Foggy Truth in play, that makes things harder for the attacker, so the target gets an extra success on their roll, which makes it harder for the attacker to hit.
Contacts must be gained through roleplay, and normally as a result of an adventure or similar significant activity - they’re not just people you know, but people who can do things for you (and will expect you to do things in return in order to keep them) on a fairly reliable basis, so they’re special. The courier talent is an easier way to get extra contacts, representing that they come into contact with a lot of people as a matter of course, but you can’t take a talent more than once unless it specifically says you can.
So, no hard limit, but they should be mainly awarded as rewards for the PCs rather than given out freely, and the more contacts you have, the more work you’ll have to put in to keep them all happy…
More or less. An Adventure might be thought of as akin to a single level in one of the video games (go to X location and achieve Y objective), or an episode in a tv series, or a chapter of a book. They may vary in length, but they should broadly be a self-contained chunk of story (with room to have loose plot threads leading into later adventures).
As written, Dishonored is more freeform and conversational with action order than other 2d20 System games, where combat isn’t really treated any differently to non-combat (i.e., there’s no fixed turn sequence). So, the GM should always be moving around the scene asking players what they do and interrupting that with NPC action in a fairly organic way. Essentially, the initiative order is “GM chooses”… and that means you can technically use any method you want (though if you’re looking for a tiebreaker, the character with the highest Quickly style is a good one to act first).
So, if you’re comfortable with the action order system used in, say, Star Trek Adventures, feel free to adopt that - you won’t break anything by including it, and it can provide structure to groups who might not necessary be comfortable going quite so freeform in a fight.
It can be viewed as referring to upgrades, but general improvements in craftsmanship (i.e., getting a hand-made, master-crafted sword might be better - but more costly - than the factory mass-produced blades used by the city watch) can also be reflected in higher damage (the blade is sturdier, better balanced, sharper, etc., all meaning it’s more effective at hurting people).