Often, first author is considered the primary contributor, while last author might be considered a principal or adviser. By adding a disclaimer and listing authors alphabetically, you can try and dispel that misconception if this isn't the case for your paper.

Typically most credit goes to first and last author. However, there are cases where one person needs to be first author, but multiple people are equally deserving.

This strongly depends on what field you are in and can vary even between minor sub-disciplines. Also that statement is frequently meaningless and authors get added for politics target than contribution.