2. No it doesn't. Let's analyse your example: Everyone, including the staff, [is/are] required to wear shoes. "Everyone" is the main bit of detail, and the main clause is everyone is required. If you remove including the staff from the sentence then you would obviously use "is". Since including the staff is somewhat separate to the main clause ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.