> IMHO the only questionable pilot decision here is to divert to Edinburgh rather than Manchester immediately.

The decision will have been made based on the forecast weather at Edinburgh prior to the flight (that is used to select a suitable alternate), and the actual reported weather at the time. Both the forecast and actual weather are precisely reported in an aviation weather language ("TAF" and "METAR") and assessed objectively. The investigation will certainly consider if the pilots erred there. Mostly likely the outcome will be that the decision was the correct one given the weather information they had available to them - this is what has been found in similar previous incidents.