>>Besides the obvious (one's a worker and the other is an old or injured queen)...

A drone-laying queen is either an unmated virgin that missed her mating window due to bad weather, or a lack of drones, or an old queen that has run out of semen.

Laying workers are regular workers that, in the absence of queen scent, developed their ovaries and started laying unfertile eggs.  [Un]fortunately, these workers don't mate.

Since you put cells in 250 de-queened hives followed by bad weather, you had drone-laying queens unless some virgins turned out badly and hives developed laying workers.  A virgin queen only has something like a 2-week window to mate before the door closes. 

>>horrendous amount of regular worker cells being converted in the middle of two frames to drone...

This again points to drone-laying queens.  In my experience, a drone-laying queen will keep a fairly tight egg pattern whereas eggs from laying workers tend to be scattered.

A drone layer will generally affix the eggs to the bottom of the cells whereas laying workers will attach theirs to the cell wall.

>>Requeening yet again solved our problem but can we be sure we didn't have a laying worker laying all those drones?

Fairly sure.  There are typically several laying workers in a colony.  Each occupying a different part of a frame or hive with her own camp of supporters.  Supporters of one laying worker will often attack and kill another laying worker.  I've seen this. 

Requeening a hive with laying workers is typically unsuccessful since such hives act as queenright and will attack the new queen.

I never tried requeening a hive with a drone-laying queen without first killing her.  I suppose a mated queen's scent emitting from a queen cage might win the workers' allegiance to the point where they will kill the drone layer.  Or a mated queen typically has the upper hand in her duel with a drone-layer.

>>can you have a laying worker in the same colony with an 
infertile queen?

As far as I know the answer is no or rarely.  The scent from a drone-laying queen will suppress potential laying workers' development.

Waldemar

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