After calculating the one to one correction (for late ACP refunds), the allocation for each eligible participant is less than $10 (some much less), and some don't even have accounts set up (they do not defer). Is there any de minimis ruling for such small corrections?

Section 6.02(5)(b) of Revenue Procedure 2003-44 provides a de minimus exception for corrective distributions of $50 or less. I cannot, however, find a de minimus exception specifically for corrective allocations. You may be able to take advantage of 6.02(5)(b) if any of the participants owed an allocation have already terminated and the correction would require a distribution. Don't know if this is bending the rules too much here though.