A regular pentagon has straight sides and is closed so its a polygon. Any shape with straight sides and is closed is a polygon. Example O (circle) has curved lines so its not a polygon L (square) is open therefore it is not a polygon.
It is impossible to have a two-sided regular polygon with equal length sides. The two sides would merely form an open angle, with no closed third side.
A side with two sides open pours more easily.
Yes, they can have sides and vertices.
A line cannot intersect in space. Space is a wide open space.