From the way you describe yourself there are many seminaries that seem like they could have been right for you. In even the most left of the MO seminaries there are still mixes where you will find every kind of girl. Even if not, around winter vacation the girls begin changing and you’ll see some girls becoming more lax with the rules and you’ll see others who start to wear the four inches below the knee skirts.
I went to a very MO seminary and I remember whenever the girls would play non jewish music they would always ask everyone in the room if they were okay listening to non jewish music.
You never have to be like everyone else. Work on making the experience positive for yourself and you’ll see that there is so much to gain. Whether the other girls are too modern or too chumradik for your taste, you’ll see that when you start respecting them for who they are as people, instead of judging them, you will gain respect and friendship. My closest friends range from the most frum chassidish, to almost ‘greasy’ yeshivish, to not observant, and if you’re able to see the character inside, you can relate to everyone. I also knew barely anything about the pop culture (I lacked the interest) yet my friends were of the most secular ones in the seminary. You don’t have to know the background behind every conversation to be able to relate to the other girls.
You sound like you’ve been working hard to find a hashkafa and community where Judaism can work for you. But you also sound a little closeminded. Try to gain the good parts the seminary and the girls have to offer you instead of focusing on the bad. Also, I bet if you ask, the teachers will give you a halachik source for why they cover their hair as much (as little?) as they do.
Why do you want to do sherut leumi? Because you love the land and want to give everything to your people or because you want to move to israel and if you do you have to do sherut leumi?
Unless it’s because you love the land and want to do as much as you can for am yisrael, if you wait until you’re 19 to make aliya, you don’t have to serve. Regarding the question between bat ami and agudah I would think the only difference is the community where they set you up.