Luxuriousness is a real word. It means the degree of luxury within something, but it is not commonly used, as it is rather superfluous.

Opulence means richness or luxury. I was amazed at the opulence of the bank once I went inside the lobby.

There is only one correct spelling for the word luxury. I could also capitalize it. LUXURY. Synonyms exist (opulence is the closest) but that is not the same thing as an alternate spelling.

Yes, the verb form of "opulent" is "opulence." It is used to describe the action of demonstrating wealth or luxury.

luxuriousness