I agree with what you say, but couldn't police have evacuated the acft anyway and left it for a couple of hours before approaching? I see no logical explanation for what happened. They can protect both their officers - which is understandably a concern, but also pax.
I have to say I'm particularly disappointed by IAA's washing of its hands; these are Irish registered aircraft, carrying Irish nationals. Does the National Aviation Security Council have a view on this? What would it do? Have any objections been raised at a diplomatic level?
OK, what has happened on this occasion has been the work of some pitiful nothing creating trouble ... but what if it actually happened and a bomb did go off; if this happens again, will need some policy in place. Will need to seek agreement from UK authorities that either (a) Irish registered acft able to get to Irish airport and issue will be dealt with professionally there, OR (b) the issue will be dealt with in the proper way at a UK airport (and the nearest one, too - not diverting hundreds of miles to PIK).
It's time for Dept of Transport/IAA to develop a backbone on this issue. The IAA has a responsibility, whether it wants to carry it out or not; it can't simply wash its hands of it.