Repeating salaah if one found out that wudhu was not valid

Q: I was sitting after performing salaah when I noticed that there is a black, sticky substance on my foot through which water could not pass. It was the size of a whole round fingernail, and I had not seen it before.

Then, my brother told me that he saw it since 3 or 4 days on my foot. I had also led jamaat during these days. Will I have to repeat all the salaah that I had offered on these days? Will those who followed me in jamaat also have to repeat their salaah?

Husband says to wife, “If you ever again say to me ‘this is your fault/this is my fault’, then Allah’s qasam I will switch off my phone and will never open it again.”

Q: Husband says to wife, “If you ever again say to me ‘this is your fault/this is my fault’, then Allah’s qasam I will switch off my phone and will never open it again.” If wife ever utters those statements in future and the oath breaks, will the husband have to pay kaffarah or what will he have to do? Since it is not possible to switch off the mobile forever.

Breaking a promise

Q: If someone said "I promise I won't repeat this namaaz again or break it, Ya Allah please help me" then they repeated the namaz they didn't have any specific niyyat when saying Ya Allah but they were aware they were saying Allahs name. Do they have to give kaffarah?

Muslim child celebrating Halloween in school

Q: In school, the teacher and students celebrate Halloween. Can a Muslim child (7 years) celebrate Halloween with the teacher and other students? Can the Muslim kids just celebrate it for fun? Is it kufr to celebrate Halloween?

If the teacher gives some gift to the Muslim student, for example, some sticker to colour and a pumpkin mask for Halloween, can the kids play with it (the kids don't know about Halloween)?