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.
The answers to your questions will be summarized as follows:
1- A person in a state of Ihraam is prohibited from wearing form fitting clothes that are sewn to fit the body or limbs. As regards wearing two Ihraams one on top of the other, whether or not they are sewn to each other, then this is permissible in our view as this is not considered as form fitting clothes which a pilgrim is prohibited from wearing. It is also permissible to carry a backpack and wrap something around the neck when sleeping, as all this is not included in the form fitting clothes which a pilgrim is prohibited from wearing.
2- As regards wearing a medical mask over the mouth and neck, then it is better for a pilgrim not to wear it, as this is included in the prohibitions which a pilgrim should not do, according to the view of many jurists . This is the view of the Hanafi, and Maaliki Schools of jurisprudence, as regards the Shaafi'i and Hanbali Schools of jurisprudence, even though they permitted a pilgrim to cover his face, yet this cannot not apply to the masks worn on the face as they do not only cover the face, but they are fitted around the head or ears with a string, and there is a difference between simply covering the face and fitting around the limb and wearing something. Since these masks are included in the prohibitions of Ihraam, then the one who wears them is obliged to offer a sacrifice.
3- Spending the night at Mina during the nights of Tashreeq, namely the 10, 11 and 12 of Thul-Hijjah, is one of the obligations of Hajj as we mentioned in Fatwa 89360. Therefore, you are obliged to spend these nights in Mina.
4- It is forbidden for a man to wear socks when in the state of Ihraam as they are form fitting clothes for one's feet. If one wears them, he is obliged to offer a sacrifice because this is one of the prohibitions of Ihraam. As regards women, they are obliged to cover their feet with socks and the like because their feet are a part of their 'Awrah (parts of the body that must be covered in Islam).
Finally, it should be noted that deliberately committing one of the prohibitions of Ihraam without a necessity or a dire need is a sin, and sinning at this time [during Hajj] and place [Makkah] is not like sinning at other times and places (i.e. it is graver).