Yes. For detailed (US) information, see the link below.
Architectural works are protected by copyright.
Yes; architectural works are protected.
Inventions are protected by patent law.
Architectural drawings and diagrams are protected as works of art, as they tend to involve a significant amount of creativity and originality.
It is, almost certainly, registered as a trademark.
Ideas cannot be protected by copyright, only the expression of the ideas.
Aspects of the design that are entirely your original work, such as a piped or carved design, may be protected by copyright as a work of visual art; the US Copyright Office has asserted that permanence is not required for protection.
The US Copyright Office limits protection in architectural works to "structures that are habitable by humans and intended to be both permanent and stationary, such as houses and office buildings and other permanent and stationary structures designed for human occupancy," so a literal reading of that would mean the grandstand could be protected but the track itself couldn't.The design of the track, as a drawing, could be protected as visual art.
They aren't; if they are of a particularly novel and non-obvious design, they may be protected by patent law.
Design can be protected by a design patent, and often copyright. It depends on whether the design is more highly technical (as computer chip design) or artistic (as a wallpaper design).
The Berne Convention, which most countries' copyright laws are based on, does not distinguish between types of creative work. In the US, the applicable law is US Code Title 17, and in the UK, it's the Copyright Designs and Patents Act 1988.
Under USA copyright law, fashion designs are not copyrightable. Portions of a product that contain "creative works of art" may be protected, including a printed design. Many types of fashion products are protected with design patents for their ornamental appearance.