Snowbunny, you sound like such a special person! I wish I could get to know you – really! It is unclear from your post (or at least I didn’t “chap” it) – are you currently in EY or in the US?
Please forgive me if giving outright advice seems forward of me, I am only doing it because I care: If you are here in EY now, and you can find a Rav here who can guide you, I think it might make your life and your decision-making much easier. There are Rabbonim like R’ Zecharya Greenwald and R’ Meisels who are very caring people and who have experience with a very broad spectrum of girls (so you won’t have to feel that they might box you in). Their numbers can be found in any Yerushalayim phone book and probably in the Newcomers Guide, as well.
If helping and giving are what you are looking for in Sherut Leumi, there are plenty of places like Ezer MiZion, Yad Sarah and Zichron Menachem who are happy to get volunteers. As far as room and board, there are also families who have girls board with them in exchange for help with the kids, something that might also have the added advantage of getting an “inside look.”
Again, you sound like a very special person who has a tremendous amount of potential. May Hashem bless you with hatzlachah in all you do!!