TG,
I tend to agree with you re. the AAIB report. If it's of any help ;
In a typical male, with normal liver function, 5 units will take 5 hours and around 40 minutes to be processed - I.e. back to an undetectable level in blood, breath and urine.
Excluding the fact that the body can produce it's own alcohol up to a level of around 20milligrammes / 100ml of blood (1/4 of the UK Drink Drive limit), I would be very surprised if any amount of alcohol would have been detectable bearing in mind the duration of the rest period (taking into account the end time of the meal/alcohol consumption).
For sure, to a limited extent, tiredness can have an effect, the type of alcohol consumed, and the fact that the alcohol was consumed with a meal.
I can't comment on the way in which the diphenhydramine is processed by the body or what effects it may have produced at the levels detected.
It is certainly in the family of medicines which can increase the affects of alcohol. However, in light of what I have written above I don't see how the two can be related.
A very sad read, and I also echo your condolences.