After you leave the shack, Astrid becomes essential yes, and you can't reset it. If you have some earlyer saves...but i guess you would have tried it. What did you think it would happen when you joined an assassin's guild? Ofcourse they are a$$holes, they're murderers and cutthroats. Try finishing the questline and see if the final stage satysfies you. And save often while you do it. Or see the UESP for spoilers and decide your course.