Fasting

You have to calculate them and add a couple of days to be on the safe side and fast them bit by bit.

https://m.youtube.com/watch?v=KPRfPKx5J34 [1]

The day of Arafat is the 9th of the month, regardless of the country you are in! Therefore, if according to the moon sighting, your ninth is on Monday, this is the day you fast, regardless of the Muslims in Saudi Arabia doing it a day earlier.

They have committed a major sin and Allah will store for them an appropriate torment in the Day of Judgement.

Yes, she can combine the niyyah for both, intending that it is for the missed day of Ramadan.

No

This is permissible.

It has no effect on your fasting at all.

No! The feeding must be to a poor person who you wouldn’t benefit from feeding.

No problem