Fasting

You must not attend this gathering as it is clear innovation. However you can eat from the food if they bring you some.

Yes, you will get the reward in sha Allah.

The fasting is valid, however he must take ghusl and repeat the prayers he had done until he realizes it.

No, you cannot fast that day. You can start fasting after you see the sign of purity that is white thread like discharge or sometimes no discharge at all.

If this Muslim is exempted from fasting, this is permissible but one must not do that especially if this hurts the other Muslim feelings.

Yes

This is permissible.

You have to follow the country that you live in.

No, it wouldn’t be wrong unless the money is specificity given for fasting people. Usually, you can’t verify who is fasting and who isn’t.

This is a baseless innovation and it is not true.