Cosford is home to the National Cold War Exhibition, which is why the money was available for the new hangar and why there are a variety of non-RAF types in that particular location (and, indeed, a number of vehicles). The Soviet kit, the F-111, the MH-53 reside there.
The Neptune is appropriate at Cosford because of the type's connection with Coastal Command (and let us not forget 1453 Flt either); likewise the Catalina. Both came from overseas and have been left in the markings of their last owners/donors. In the case of the Dutch, the squadrons operating the Neptune were 320 and 321, which were RAF units in WW2, both of them serving in the MR role (albeit in 320's case not for the whole war), which adds to the Neptune's presence being appropriate (the Danes didn't have quite enough pilots for their own RAF 300/400-series squadron, but did attempt to form one.)
A further key point is that Cosford was the Aerospace Museum for more than the first decade of its existence, with a clear remit to cover not just the RAF's history (the early 80s Cosford visitors' guides, by the by, talk of Hendon as being a national aviation museum and 'devoted...to the complete story of the RAF' with Cosford as an 'aeronautical collection'). This is why there was a BA collection - now gone, of course - at the museum for many years as well.
This means that a number of the 'questionable exhibits' not in the Cold War exhibition are there because Cosford was the right place for them under its original remit and they are now (e.g. the Ki-100) seen as being of historic significance in their own right. It was also appropriate for aircraft from the opposition during WW2 to be displayed - is it better that the actual items are on display so that the aircraft which the RAF (and FAA) went up against can be appreciated in 1:1 scale, or should they be slightly faded photographs up on a display board, thus maintaining some sort of 'purity' to the collection?
Cosford isn't creating a BoB museum by having the Dornier (which drew in hundreds of extra visitors in the first week it was there, and is still popular) - it is there because that is where the conservation centre for the RAFM is located and it is rather in need of a spot of conservation, even if it isn't going to be rebuilt. Once the airframe has been brought into a condition where the bits can be displayed without needing the special conditions they currently reside in, the plan is - or was - to ship it down to the BoB Hall at Hendon (the Hall, of course, is to all intents and purposes a BoB Museum, but it just happens to be within a bigger museum).
As for:
But one could question what the RAFM is doing creating (sic) a BofB Museum.
Perhaps they were under the mistaken impression that the Battle of Britain was a rather important part of the RAF's history and the battle with which the public most associate the RAF?
I agree that it would be appropriate for XT597 to end up at Cosford as part of the collection; it scores both in terms of the Cold War angle and its use for testing and evaluation.
But to blame it on foreign types being preserved, and to question the validity of those types being there is pushing it, I fear, particularly when the way in which both Hendon and Cosford evolved is taken into account. Unless Qinetiq offered XT597 to the RAFM and they declined it, then blame Qinetiq for disposing of it in the way they have (although thinking about it, didn't the US previously object to Phantoms being disposed of through the likes of Everett and insist that redundant airframes were scrapped instead?)