Is there such a thing as a statute of limitation on ADP/ACP testing? I remember hearing something about a 3 year period but am unsure if I'm inventing this.

You can correct until the end of the following plan year under the regs, and then you can self-correct for the next 2 years under VCP (since the operational failure is considered significant)--hence, 3 years. After that, you can use VCP to correct.