Yes, that is interesting to have a more scientific version of why and how the problem arises, so we can make up our own minds as to what to do. Ultimately, the decision as to whether the fleece is spun/felted/whatever with recent Crovect on is up to the person doing the work, shearing, washing and spinning. We get around it by not using Crovect after the autumn flies have gone right up until shearing. Should a sheep be struck before we shear it, then I would discard that fleece, but I am in the position of having more fleeces than I can possibly ever use myself. Because we shear our own sheep we don't have to wait on a shearer, so in fact, fingers crossed, we've not had strike for a very long time. We use Crovect on the lambs in mid May as they are most likely to have a problem.
I hate using chemicals, but this is one for which we make an exception, as fly strike can be so awful.