You need to finish all 18 holes to complete the showdown- so YES to #3
actually you don't. but it depends on the person. if a person is comfortably within the cut line of the elimination round, and can absord 3 bogies they can still qualify and advance to the final round without playing all 18 holes.
if you reach the finals - and you stand no chance of advancing further - or cracking the top 25 or top 15 - then again - you could simply save the time and effort - receive bogies and you still receive essentially same type of prizes if you complete the round because the differences in the bottom half of the leaderboard are only marginally different.
some people won't even play or forget to play the final round at all - and they still receive their free ball for reaching the finals from what I have seen.