What is Online Adventure mode?
In Online Adventure you compete against other players in a series of races. Earn XP by racing fast, clean, and with style. After all the races are completed, the player with the highest XP total wins. You can matchmake into Racing only (Collisions On or Off) or Games only through the Custom Adventure option, or choose Online Adventure, which is Racing and Games combined.
Is there a Spectate feature for players to watch instead of race?
No
How are races scored?
The person who crosses the finish line first is always the winner in Horizon MP races. There is also a separate XP championship running alongside races where those who don't cross the line in the top can still accomplish something.
(Written by Manteomax)