Fasting

If it was done mistakenly, the fasting is valid.

She can combine the intention for the missed Ramadan fast with Ashura.

No. Kindly refer to the following link(s): https://m.youtube.com/watch?v=20vzfCswEAs

This is counted as Ramadan.

Yes

This is permissible

If you didn’t fast because of menses it is sufficient to make your fast. If you didn’t fast due to illness, and you have chronic disease, you  have to give kaffara for one meal.

It is not permissible to fast on Eid and on the 11th, 12th and 13th of dhul hijja. You can resume fasting on the 14th and 15th (white days)

One must stop eating and drinking in the first word of athan. The only exception is if he was holding the glass or the bottle and the athan begins, he can drink as much as he wants from it.

In this month, it is prohibited to fast the 13th. You fast 14 and 15 and this is an exception