What is the ruling on continuing to attend a mosque where decisions are made by a committee, some of whose members do not adhere to the Sunnah and Sharia and commit sins, and where women do not observe the correct hijab, or is it legislated to establish a new prayer hall?

You should continue going to the mosque and strive to improve things from within through admonition, advice, and better relations, without engaging in forbidden acts. Written suggestions presented gently and kindly may contribute to improving conditions, and be optimistic about things getting better in the future.