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The following are the answers to your questions:
1- It is makruh for one who is junub or haiz to use henna.
2- Washing with plain water suffices.
3- In wudu, it is necessary that the hands, from the elbows to the tip of the fingers, be washed completely. Therefore, the nails and a normal amount that has grown on them (2-3 mm) are considered part of the hands and must be washed. So all things that might interfere with water reaching them (i.e. nail polish) must be removed first.
For further information, you can refer to the following:
1- Nail polish and Jabirah wudu, Question 4680 (website: 4966).
2- Nail Polish and wudu, Question 2448 (website: 2578).
Ayatullah Hadavi’s response to the question:
1- Washing the nails that are in the hand area is wajib and if there is anything blocking them, it must removed beforehand.
Question 1960 (website: 1985) The body being pak while performing ghusl
 Tawdihul-Masa’el (annotated by Imam Khomeini), vol. 1, pg. 214, issue 356.
 Tawdihul-Masa’el (annotated by Imam Khomeini), vol. 1, pg. 270.
 Nijatul-Ibad (Imam Khomeini), pg. 19, issue 5: “After washing the face the right hand must be washed and then the left had from the elbow to the tip of the fingers, and in order to be sure that the elbows have been completely washed, he/she must wash a little higher than the elbows.”
 Adopted from Question 5779 (website: 6010).