Aer Lingus have being refused promission to land a KTW a number of times. Ryanair will get top priority from KTW as will LOT. I was on a Ryanair flight and around 12 flights diverted to KTW and there was no bus transfer to KRK as they hadn't enough to take 12 loads of passengers.
That is interesting without doubt, however surely there are other options such as Poznan (Previously served by EI), Gdansk, Wroclaw etc. Of course they're not ideal but they're a lot better option than Berlin. In my experience LOT and Wizz are top priorities at Katowice followed by Ryanair, seeing as the former two have a base there. In truth though they shouldn't be cherry picking based on airline. More it should be first of all flights which are originally due to land there, then those with the lowest fuel and then there should not be any real favoritism.
Warsaw is used a lot for KRK diversions but again they have also being refused promission to land. In most cases if KRK has problems with weather other airports in Poland do to. Its up to KRK airport to actualy spend money on instaling equipment to stop this.
From my experience on FR at least, then generally tend to use the nearest open airport to the one that is closed in diversions Europe over, regardless of if it's regularly served. Although they, and Wizz Air have had some problems with Warsaw (Chopin) in the last few weeks when it comes to handling diverted flights from Modlin, with planes getting stuck in Chopin and being denied permission to leave for hours and hours. I agree though that flag carriers tend to always go for WAW for KRK diversions rather than KTW.
I do agree however that KRK needs an upgrade of infrastructure, the terminal is bursting at the seems, there is only one baggage belt and it's crazy overcrowded and that is before you get to deal with the lighting situation. Ideally all of the airports need to have some different ownership structure, as right now one company has a large share in pretty much every airport. KRK and WMI are in terrible locations for fog and for this reason PPL should pull it's finger out and provide proper infrastructure for all it's airports, rather than just WAW.
Berlin is used as in most cases there is no other option.
AL made it clear on many occasions (at least in 2010 and 2011) that their procedure is always to land at an airport that is served by EI as this would prove to be a better outcome for passengers. They did not consider other airports.