A great niece and a grandson of the same person are second cousins to each other.
There probably isn't a relationship. You do not share a common ancestor.
Your great uncle's niece could be your mother, or your aunt. It is also possible that she is not related to you at all, as you might not share a common ancestor.
Your great aunt is the sister of your grandparent Her son's son is her grandson. Her grandson and you share a great grandparent and thus are Second Cousins to each other.
She has no relationship with you. You do not share a common ancestor. You technically are not even related to your niece's husband.
Your husband's cousin's grandson is not related to you at all. You do not share a common ancestor.
Your second cousin and your grandson are second cousins, twice removed.
A niece or nephew is the child of an aunt's or uncle's brother or sister. So the parents of the aunt/uncle and of the niece's/nephew's parents are the same people - their grandparents. The grandmother of the aunt or uncle is the great grandparent of the niece or nephew.
There may be no relationship. If you do not share a common ancestor, you are not technically related.
You aren't. You do not share a common ancestor.
Yes, it's possible. Because the niece's father and the husband share the same father so any offspring of the husband would be related. There is no specific name for this relationship, but presuming no illegitimacy or adoption, there is a genetic connection.
There is no issue, as it is very unlikely that you share a common ancestor, there is no law to prevent it.
Yes , your great aunt and your niece will share some but not all of your DNA. In fact, you will probably share much less than half with either of the, That is, of course, assuming that your great aunt is directly related to you, and not your aunt by virtue of being married to your great uncle.