1. Is it correct?
Yes, it is correct.
Use case 1: Using parentheses to indicate words of less importance such as during a digression or mumbling.
"... I do assure you that I am not one of those young ladies (if such young ladies there are) who are so daring as to risk their happiness on the chance of being asked a second time."
Jane Austen, Pride and Prejudice
Use case 2: Using parentheses to indicate action of the speaker or present some other information not spoken by the speaker.
"... Yes, I understand the sort of mind. Vigorous, decided, with sentiments to a certain point, not coarse. A better written letter, Harriet, (returning it,) than I had expected."
Jane Austen, Emma
Here's another one from a modern book:
"... You an see her padding feet and feel the stink of blood on the breath. (Meanwhile he Henry Wyatt, in a cold lather of fear, backs off, backs away, in the direction of help.) In his soft enchanting voice ..."
Hilary Mantel, Wolf Hall
2. Is it usual?
Parentheses inside quotes are not usual.
In Use case 1, commas are preferred, and in Use case 2, it is preferred to break the dialogue is and convey the additional information in the "dialogue tag".
The example from Wolf Hall is an excerpt from a long dialogue; the character is telling a story to others. I can see why the author didn't want to break the dialogue and, so, she used parentheses.
Using parentheses inside dialogue such as in Use case 2 can be a useful tool to the author. However, overusing it may seem tacky, so it's probably best to save it for one or two really special cases.