One would hope EU261 would never apply in the event of a medical diversion. Should a passenger claim compensation under EU261 for a delay due to a medical diversion then they are the lowest of the low. Not to mention the extra financial impact on the airlines and potential internal pressures not to divert due to medical issues.
If this were to happen on the outbound flight of the first wave for a short haul airline then then you're looking at over €250,000 in compensation based on six flights. Madness.