Thanks for writing. I am sorry you are having trouble.
The reason the grey "unknown" shows up when you try to delete someone has to do with the way family trees are built and how the family tree program at MyHeritage works.
In standard family tree logic there must always be a man-woman pair whenever a child exists in the tree.
In order to comply with those standards, the family tree program at MyHeritage also requires a man-woman pair in order for a child to exist in the tree.
This means that anytime there is a child attached to two persons in the tree, and you try to delete one of those parents, the program is forced to keep a "person" in that place.
We are currently working on a better solution to this problem.
In the meantime, your best workaround is to make sure that there are no children attached to a person that you would like to delete.
I understand if this is frustrating for you because, for example, there may be persons "high up" in the tree that you need to delete and it would mean deleting a lot of children in order to get to him/her. The best thing to do is EDIT the information on that person if you can rather than trying to delete him/her altogether.
If you have further questions, please let me know.