For mahrams ONLY.
They shouldn't cake themselves in front of non-mahrams, no matter how modestly they dress.
Yes and yes.
The 4 Imams, Ibn Qayim, Ibn Taymiyyah, Al Bukhari, Muslim, Ibn Kathir, Al Tabari, Ibn Baz, and most of all, Rasulullah (Peace Be Upon Him) all said this.
Not a single reputable Islamic figure says that the Hijab is not mandatory. You cannot come in with your brand new interpretation and say that everyone else is wrong. These people have spent more time learning Islam than you have probably even been alive.
As for jamahir, I don't really care about his input on Islam since he openly denies parts of the Quran.