Carson City

Yes — you may place a cowboy on a square or parcel already occupied; that triggers a duel in which each involved player rolls a die and adds their firepower (cowboys + revolvers in reserve); the highest total wins the right to perform the action and losers return their cowboy to their personal reserve (tie broken by lower turn-order number). (p. 3)

Yes — some squares explicitly allow multiple cowboys without a duel: Wages gives $4 per cowboy on the square, and one Roads square gives one road per cowboy on it; other action squares follow the board's sequence and duel rules. (pp. 3–4)

Generally a parcel must be reached by a road (at least one road touching a corner) to build, and many buildings also force you to place a house on a free road-reached parcel; exceptions: ranches and mines do not need a road. You may also buy a building and keep it in front of you to place later (see timing rules). (pp. 4, 6)

Any money above your personality's cash limit must be spent; for every $10 you must spend you receive 1 victory point (you may spend more money voluntarily and still get 1 VP per $10). Each personality card shows its cash limit. (pp. 3, 7)

An owner-free house increases neighbouring buildings' values for everyone, but a house built on a parcel belonging to a player only benefits that player's buildings (the same distinction applies to mountains/mines). (p. 5)

No — you may not attack a church or a prison; furthermore a church prevents attacks on all directly adjacent buildings of its owner, and building a church next to a building being attacked cancels that attack. (p. 6)