I really feel for you! It honestly does sound like you are way more invested in the friendship than she is.
My wedding is less than a month and a half away and my "unresponsive bridesmaid" has not only slyly backed out of the wedding using excuse after excuse, but she has been 110% uninterested and uninvolved in my shower and bachelorette party. Every time I have asked her point blank, she just gives excuse after excuse. My shower is this weekend and she never RSVPed and texted me the longest excuse ever after I asked her what was going on- she never even gave me a yes or no answer, so I've cut my losses. I don't need to wear myself down by trying to prove myself to someone who couldnt care less.
I would take a step back and look at it from a broader perspective. I bet you she treats other people this way as well. I believe in karma and know these people will regret acting this way to others, but by then it will be too late. It might also be easier for me since she lives in another state.
If I were you, I would give her an opportunity so she can't say you didnt ask her, but just assume that she will have nothing to do with it. Attend her wedding of course, but I wouldnt go out of my way for her anymore. It's just hurting you more in the end. It is what it is!
Also with your destination wedding, I said the SAME EXACT thing about giving people almost 2 years notice to save. It doesn't matter in the end, because some people are just not good with money management. People will go who you never expected, and the people who you thought were definites will back out. Just a week ago, my two friends who do hair and make-up backed out. Have a backup plan for everything and expect nothing from anyone! I know that sounds bitter, but it's actually helped me from feeling so hurt when I get next to nothing from all these friends who I've gone out of my way for in the past...