The city has been awash in heavily armed security officers and volunteers - many of them retirees dressed in yellow golf shirts who stand sentinel on nearly every street corner - for the huge military parade planned for Thursday.
2. A parenthetical structure can be (almost) any grammatical structure. "Parenthesis" merely explains where it is positioned, and that is an afterthought, or an interpolated piece of information.
The term "almost" is my sentence above is an adverb. It is also parenthetical. The terms describe different things - one has no bearing on the other.
The phrase in red is a relative clause. It is parenthetical in my awkward rephrasing, but not in the original.