Puzzle
You raise your question in terms of a puzzle. Glaucon, to you, sounds incoherent in his conception of luxury. On the one hand, it is the source of the evil (wanting more than what is necessary to live a good life). On the other hand, since many suggested luxury items (sofas, tables to dine on, sauces and sweets, which Athens then are accustomed to, a la Glaucon) are ordinary conveniences of life, luxury cannot be the source of the evil. You wonder among others, whether there is a way to reconcile this two seeming incoherent conceptions of luxury.
Naturally, yours is an interpretative question, meaning that there can be other good interpretations depending on what strikes as the salient feature in the text. For this post, I use http://classics.mit.edu/Plato/republic.3.ii.html. This is my way of solving the puzzle.
The two stages of the state
Luxury becomes a passing focus because Socrates and Glaucon were discussing what are the essential elements of a just society. Socrates, in particular, wants to establish the need of the class of solders (warriors) in the just society.
Both Rawls and Socrates want to know the nature of a just society. Rawls' method was to deploy a hypothetical contract situation. Socrates' method is a pseudo-naturalistic-empirical explanation of how a village grows into a city-state. Socrates posits two stages. The first stage is the healthy state. The origin of this state is people wanting to exchange their surplus goods. So created is the market to barter. In this society, basic necessities are met, majority leading a vegetarian life style, and meat is acquired through hunting. Socrates conjectures that people in this society would live in peace and health to a good old age.
At this point, Glaucon complains that the life of the healthy city-state cannot be comfortable since we need some conveniences of life too. Thus posited by Socrates is the next state: the luxurious state. The state now needs to support those who do not produce basic necessities of life, like poets, physicians*, rent-seekers and politicians. To support the parasitic or leisure class and their conspicuous consumption, more land is needed, which is the source of war. This is why Socrates declares that the causes of war is essentially the same as the causes of the evils of the state: the desire for (or the demand for) luxury.
Their discussion then moves on to why solder class is necessary since the need for land will be felt by all luxurious societies and thus inevitable is war.
Dissolution of the puzzle
Viewed in light of the two stages, where the luxurious nation comes after the healthy nation, which both Socrates and Glaucon believe are natural growth, Glaucon's view of luxury is not incoherent. The conveniences might look luxuries in the healthy state, but not so much (just necessities for comfortable life) in the luxurious state.
Footnote: Socrates conjectures that the luxurious state needs many physicians since the ordinary conveniences of life would render people's constitutions weak: imagine the people in Wall-E movie.
