According to Google structured data testing tool, all ListItem URLs must belong to the same domain – but my company’s domain does not have five different URLs about all items. Thus, I have put Wikipedia’s instead.

That’s a tougher error, Google’s documentation on this is a bit confusing.  Here is another article that helped clarify the problem further. In this case, the url might not be the best property. If you use Wikipedia concepts, you could use sameAs and the url would be used to point to the details of the data item with your domain.  You can also switch sub-accounts using the drop down at the top of the Schema App screen and move data items between sub-accounts using the Actions>Move to Account. Read more in the blog here.