Yes crews are required to submit a DASOR following a TCAS RA event, unless they have a reasonable expectation that ATC will submit the report (not many know about that caveat). The issue is that many aircrew submit an airprox report because they have had a TCAS RA - there is not necessarily a requirement to report an airprox because you have had a RA. RA events occur even when lateral and vertical separation is being controlled by ATC because of the way that the system is designed. An RA does not equal an airprox unless you believed that the safety of your aircraft was or may have been compromised.
The ATCO turned the aircraft onto a base leg to ensure that it remained inside the CTR and the crew were slow to respond to that request; hence the aircraft began to approach the very edge of the CTR. That said, given the relative geometry between the 2 aircraft, even if the crew had initiated the turn in a relatively quick time after the instruction was passed, it is likely that a TA would have still been initiated.
And Fabs, you're absolutely correct. The UKAB have for some time deliberately phrased reports to focus on particular aspects of an incident to paint an ATCO or pilot in a dim light. Up until recently, their favourite target was military ATC. I've observed a UKAB meeting where 75% of the discussion centered on the incorrect action of the flight crews involved and 25% on the ATCO and yet, in the subsequent analysis, around 90% of the text focussed on the UKAB.
As far as the size of the BZN CTR is concerned, the fact is that it isn't the correct size for the size of aircraft and their IFR procedural requirements, specifically vertically and requires amendment. The introduction of controlled airspace is not necessarily a bad thing and doesn't need to men restricted access to GA activity, it just needs to be the right type of airspace for the task. Should MATZ continue to be Class G with no meaning to civilian flights, given the type and volume of movements that occurs within them? Look at it on a case-by-case basis - discuss. That said, there should be a mechanism to review existing controlled airspace structures to ensure that they continue to be valid for the type and level of activity at that aerodrome - that doesn't appear to be the case at the moment.