Yet out of this mixing pot of motivations I can't see how someone could claim they were justified.

No argument from me on this point, What I eluded to was that many people of the time of the crusades found justifications for their cause.

do you think the ISIL excuse is manufactured post hoc,

If post hoc means did they start with religion as their motivation for current events, then I would also agree in principle that their idea of Islam plays a strong part in motivation, but they are an isolated group, large in numbers for sure, but isolated none the less. I'm convinced that if religion was removed as motivation, the turmoil happening there would still be dangerous, poverty and ignorance still remain as mitigating factors, we don't control parallel histories, so its rather difficult to determine what life would be like now, so we just figure out how to deal with a past religious reality.

The trouble over there has to much historical depth, to fix the dominant religions we would have to accomplish the impossible, meaning time travel and other such fantasies.