If you do a year abroad, you could graduate first or wait and graduate at the end of that year - either way would work. You could also do just a semester abroad, and take dual credit classes the other semester. Being out of the country while doing college apps can be a little complicated, but if you get a head start and complete as much as possible before you go (and have recommendations, etc. all lined up), it shouldn’t be a reason not to do it.
I’m not sure we can say that one plan or the other (studying abroad vs. taking dual credit classes) will absolutely increase your admissions chances - it really depends on how your choice fits in with the rest of your profile and interests. I see you’re already taking AP Spanish, and Spanish fluency is extremely useful for social workers and for lawyers in many areas of specialty. Building on your stated interest in child welfare, I think that studying abroad in a Spanish-speaking country and attaining the kind of fluency that this sort of immersion would confer, could make you a more interesting and appealing candidate in terms of college admissions, and could also open doors for you in the future.
Of course, you can study abroad as a college student too. But in my personal opinion, those experiences can be hit-or-miss in terms of real cultural immersion. As a high school student, you’d be part of a real community, in a way that often doesn’t happen for college students. If it’s something that genuinely interests you, I’d really encourage it - it’s really a once-in-a-lifetime opportunity. But if you’re only considering it because it would look good on your applications, I can’t necessarily say that it’s objectively better for that purpose than taking DE classes at home.
If you’re considering the study abroad option, start looking into those applications right away, and talk with your guidance counselor about the pros and cons of graduating before or after your year abroad. My sense is that if you’re not going straight to college, there’s no benefit to graduating early, but there could be factors I haven’t thought of. Just make sure that you don’t take college classes after getting your HS diploma, so that you have to apply to colleges as a transfer rather than a freshman applicant.