Am I accountable for the sin of whoever reaches accounts with forbidden content (unveiled women) because my client follows them, given that my work in montage contributes to increasing his followers, and is my work considered forbidden (haram)?

If your work and its content are permissible, then it is not forbidden for you to work, even if the content creators follow accounts that contain impermissible things according to Sharia. The impermissible things are only accounted for by their owners and those who directly and intentionally assisted them, and the questioner's work does not fall under that.