All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, may Allaah reward you for being zealous for the truth and we ask Allaah to increase you in guidance and piety, and make us and you firm on the truth.
Our answer to your questions will be summarized in the following:
1- The evidence (which we are bound to follow) is what is mentioned in the Quran and the Sunnah. Allaah Says (what means): {And if you disagree over anything, refer it to Allaah and the Messenger, if you should believe in Allaah and the Last Day.}[Quran 4:59] What people do should not be taken into account if their acts are not in conformity with the Quran and the Sunnah.
2- When doing Da’wah, the person calling people (to Allaah) should focus on establish faith in their hearts with things like reminding them of the Greatness of Allaah and the importance of obeying Allaah and His Messenger, and reminding them of death, the Day of Reckoning, and Paradise and Hellfire, as this is most likely to be accepted by people. Indeed, this was the guidance and example of the Prophet .
3- Hijab is a religious obligation and it has some conditions which the scholars mentioned; we discussed this in Fataawa 83702 and 83033. Anything that is not in conformity with these conditions is forbidden. However, you should know that it is not a condition to wear a specific dress for Hijab; any dress that fulfils these conditions is considered Hijab. Moreover, a Muslim should lower his gaze from seeing what Allaah has forbidden as a means of abiding by the Orders of Allaah. In addition to this, one should not go so often to places of afflictions, like the markets except for a necessity.
4- We have already clarified the 'Awrah (parts of the body that must be covered in Islam) of a woman in front of her Mahaarim in Fataawa 90591 and 85449. As regards her husband, he may see from her body whatever he wishes.
5- We have already clarified the 'Awrah of a woman when performing the prayer in Fataawa 83269 and 86325.
6- It is permissible for a Muslim to buy and sell such clothes you mentioned in your question but it is not permissible for him to sell them to a woman whom he predominantly thinks that she will not properly cover her body with it. However, if in general women do not cover their body properly in a particular country, then it is an obligation to avoid selling the clothes that do not cover the whole body of the woman except for whomever one thinks that she will not use it in disobeying Allaah. Consequently, the lawfulness of the income that is earned from selling these clothes depend on the permissibility or impermissibility of selling these clothes.