The laws of the UK, including marriage laws, do not apply differently depending on someone's ethnic or geographic origin.
No.
In most places, marrying your niece is highly frowned upon and usually illegal. being considered to be incest.
No
In most places, it is not legally permissible to marry your niece. Laws vary by location, so it is important to check the specific laws in your area.
If you marry your girlfriend, you niece (daughter of your brother or sister) will continue to be your niece and will also be considered by most to be your new wife's niece. Your new wife will be her aunt, as you are already her uncle.
He could not find another wife.
The niece could be the daughter of the sister-in-law's sibling and not related to you, so you could marry her. However, the niece could also be your mother or your aunt, in which case you could not marry her. Erm, but whover readwhat this guy just wrote, I expect you're wondering WHY WOULD YOU EVEN BOTHER? DON'T YOU HAVE ANYTHING BETTER TO DO??
In terms of genealogical relationships, a great niece is the child of one's niece or nephew, while a half great uncle is the half-brother of one's grandparent. Legally and socially, marriage between a great niece and a half great uncle would not be prohibited in most jurisdictions, as they are not closely related by blood. However, such a relationship may still be viewed as controversial or unconventional due to the significant age gap and familial ties.
No. Nether of his wives were related to him in any known way. ( President Benjamin Harrison married his wife's niece.)
You better hope the State where you live doesn't find out about it. Even here in Georgia, that is illegal. There is no real tax advantage in this scheme that you are asking about so I don't see any benefit.
You can just say, "YOU CAN'T MARRY HIM!!!"
No state in the US permits marriage between niece and uncle.