Hey londoncalling1,
A line in your post pretty much sums it up: "I don't think I can pass up the opportunity I have over the next 12 months".
Well...then...don't! Here's the way I'm reading your situation (and so, if I've understood incorrectly, feel free to correct me!): you have the experiences, academic history, and personality to have gotten into a school you'd love to go to. You have an opportunity ahead of you that is fully aligned with your interests and will make you an even stronger candidate! So if you could get in this year, in 12 months you should be an even stronger applicant. Theoretically, of course, but the thought isn't exactly groundless.
The admissions committee is correct - turning down the offer won't necessarily impact your chances. Of course, as you communicate with them be sure to express not only why this is the best option for you, but for them too. Does that make sense?
Look - what it comes down to in all honesty is that these are the most important years of your career. If you have this opportunity ahead of you that you really want to pursue, do so! Next year, reassess everything. Even reassess where you're applying. A lot can change in a year and where you got into this year may not even make the list next year. This can sound a little strange at a time where you just spent months applying and waiting, but it's the truth.
I hope that helps and good luck on this amazing opportunity, whatever it may be!
Bhavik