I had a long and serious talk to her. Before I wrote my previous feels post the general status was me, taken aback, deflecting the problem as "well that's your choice in the end." I mean it still is her choice, but now I've leveled with her heart to heart and told her I want her to come. It just won't be the same. I'm still going no matter what.
She's my most precious friend, and during the talk she confessed she felt angry at me for taking such a strong stance. That meant she couldn't run and hide from the problem, take the easy way out, and it upped the stakes on both sides. Now my request for her to come is just as strong as her doubt to stay home. Of course she was angry at feeling angry, etc, but we didn't try to force out a final verdict while all of the feelings were fresh and up in the air. During the talk she also recognised the fear of missing out if she doesn't go, just as you pointed out anon. She's still going to think about it.
I prematurely wallowed in defeat after she went home. She did an exchange year in school, and greatly argued for how the volunteer project will be more intense and life changing experience if I do it without anyone to rely on. It sort of felt like I had already lost this battle, and she won't come. I'm allowed to be angry at her if she lets me down. The only condition is that I won't terminate our friendship or stop believing in her. Although, I sure as hell won't. She's worth too much for me to just quit.
Here's to hoping my premature defeat proves false.