My DD is a swimmer. As a swimmer she competes individually, scores points for her team's overall standing and on a relay team of 4 girls all in the same meet. That kind of individual/team dynamic is different from the pure team dynamic in a sport like volleyball. Having blind faith in your team is easier to do when you practice and compete substituting in players. It's not so easy in an individual sport like swimming and golf with a team component.
In addition, think about your best 6 girls on your team. The way they could read each other and know where to be to get the pass and set up perfectly for the spike. Replace one girl with another and everything is little off ... just enough so the team struggles. Even with an individual sport like swimming the girls who swim together on the "A" team get to know each other's quirks. They know that one girl speeds up at the wall while another might take an extra stroke. They KNOW how the girl who swims before they do finishes and it's familiar when it comes for the relay race. They can anticipate the rate of the finish and get the fastest start possible. Put a different girl in the breaststroke for the championships (like happened to my DD's team this season *sigh*) and everything gets messed up.
The girls on the OP's DD's golf team know each other and have a rhythm. They know what to expect from every girl on the team ... how each girl hits the ball on each stroke. The new girl will mess up the team more by her being
different and her play being unexpected than her being 7th instead of 6th.
Frankly, I cannot begin to think of a single good reason for the family to arrange a graduation party on the day of the sectional tournament other than golf simply does not have the same importance with the soccer scholarship already in the girl's pocket. She already has a free ride so why bust her butt or inconvenience Uncle Jack? What makes her abandonment of her team even worse is there are two other seniors on her team that may miss out on getting a golf scholarship because Miss #6 let the team down. If the team does not go onto to the championship tournament those girls may not get the same notice from the college recruiters.
Golfgal, I would try to put Miss #6's party behind you and start to talk up the JV player to the other girls. Maybe she'll rise to the occasion and put in a good day

. Maybe the top 5 girls will do well enough that they won't need Miss I-Already-Got-Mine-#6 or even her replacement. In any case, it will be a good experience for the JV girl to participate in the tournament and it will make her a stronger player for next year. Good luck

.