All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already issued Fatwa 90042 clarifying that in principle the grave is to be used for one dead person and that it should not be dug up except for a necessity. We also clarified in the Fatwa that in case where more than one deceased person is buried in one grave, it is a condition that the first corpse should have been transformed into earth, then whatever is left from the bones of the first dead body would be gathered into one side, and the new dead would be buried in the other side, and that it is desirable to put a partition between the two.
Nonetheless, it should be mentioned that dreams do not imply any religious rulings.
Based on the above, that girl should not have been buried with that man as there was no necessity for doing so. However, since this has already happened, do not dig the grave once again as this violates the honour of the dead.
You will find answers in the above-referred Fatwa to the other questions which you asked.
Allaah Knows best.