I am reposting what I wrote for your other question (in purple) under this answer.
The words in the parentheses are part of the sentence. It is just that they can be removed without changing the meaning of the sentence.
Therefore use 'a' to agree with red not 'an' to agree with apple.
My question is why (oh why oh why) are you enclosing things this way? Even an (older) writer like me doesn't do this.
The problem is the use of parentheses. These are used around words that clarify or are used as an aside.
My mother planted lots of flowers (tulips, daffodils, begonias) in her garden.
My mother insisted (she always did) that I behave like a lady.
In the examples the words in the parentheses are neither clarifying or an aside.
Proper planning (and design) is/are crucial
A priest (and a rabbi) walk/walks into a bar
So they are incorrect.