"The Oneida is a singular noun and hence cannot take the plural verb 'were' " is not necessarily true.
Take this question as an example:
In addition to being China's first administrators, in the sense that they developed a coherent bureaucracy for their empire, the first literate culture in East Asia were the Shang, and they were well known for crafting ornate bronze ritual vessels.
• A the first literate culture in East Asia were the Shang, and they were well known for crafting
• B the first literate culture in East Asia were the Shang, well known as the crafters of
• C the Shang, as the first literate culture in East Asia, is well known for its crafting
• D the Shang were the first literate culture in East Asia and are well known as the crafters of
• E the Shang were the first literate culture in East Asia and well known for its crafted
The correct answer is D