Faraaidh of wudhu

Q: My father is elderly and has difficulty lifting his feet to the sink when performing wudhu. Is it permissible for him to wash them by placing them in a bowl of water? 

Q:

1. After making wudhu, dirt on the cheek remains even after washing the cheek, is the wudhu valid or must the that part of the face be washed again?

2. After making wudhu, sunblock on the face remains even after washing that part of the face, is the wudhu valid?

3. After making wudhu, the dirt (like sand which you normally find when awakening from sleep) remains in the corner of the eye even after washing that part of the face in wudhu, is the wudhu valid?

Q: I know that anything that stops water from entering your skin is haraam (nail polish, wax, etc). But if I only do white nail tips, or any colour nail polish on the tips of my nails which isn't touching the skin. Is this ok!

Q: If we perform Salaah behind an Imam who did not make masah of the neck and did not wash his feet. He only made masah on his ordinary cotton socks.  Will the Salaah of the Imaam be valid and will the Salaah of the muqtadis be valid?

Q: Is the eye mucus permeable when it is soft? It looks impermeable when it is hard.

Q: I prayed Fajr and was clearing up my room and about 40 minutes later I saw something on the tip of my thumb's nail and some bit inside. So I put it under the tap to test it. The bit inside was gone, but there was still some blackish mark on the tip of my thumb's nail, but not on the skin - I believe. So should I make up that prayer or not? Looks likely to be impermeable, but it's on the tip of the nail anyway. Literally the edge.

Q: Does a full sunnah length beard have to be completely wet during wudu?

Q: Is it necessary to remove any rings and bangles when making wudhu?

Q: Is it necessary for one to repeat the masah of the head in the case where one had his hair cut after making wudhu?

Q: Is it necessary to wash the area between the sideburns and ears in wudhu?