An apostrophe after a closed parenthesis is definitely wrong if you are asking about English, although I can't say anything about legal conventions, which are often weird, so I'm answering about English. You make the sentence correct without the parenthetical phrase: "Plantiffs' Interrogatories". Then you add the parenthetical phrase. Since you're saying that typically you don't match number (as in "Plaintiffs (Smith)" -- your example), you don't do it here either, and I'd skip the possessive as well: "Plaintiffs' (Smith) Interrogatories", That is, the parenthetical phrase specifies who the plaintiffs are, and doesn't need to formally act in the sentence interchangeably for the word "Plaintiffs'". The point here, as in all writing, is to convey the meaning as efficiently as possible without distractions. Wacky use of apostrophes and parentheses definitely would be distracting.