Will the DOL accept an IRS private letter ruling that a plan is a "church plan" under Code Section 414(e) as authority for the plan to stop filing Form 5500s, or is a DOL opinion letter required also?

I don't have an answer for you, but if you're looking prospectively it may be a moot point because I've heard the IRS has a moratorium on issuing new church plan rulings where the employher itself is not a church. On the other hand, if the employer is a church, and it has an IRS ruling, as a practical matter how likely is it that the DOL would take a different view?

Your suggestion to get IRS and possibly also a DOL letter seems way overboard. If no 5500 was needed in the first place (and presumably filings should be rejected for nonelecting church plans), I'd think you should just call the DOL to get their input or just right a letter. I'd just say in the letter that you're a church plan and that you mistakenly filed the 5500 based on incorrect advice from [fill in the blank].