1) The reason for him to have become irresponsible is unknown.
Seems like he has become irresponsible and we don't know why.
2) The reason for him to become irresponsible is unknown.
Does it mean that we don't know the reason why he could become irresponsible in future?
Is it possible that the second could be said by a native speaker with the intended meaning of the first one?
I think we should clearly differentiate "idiomaticity" from "the pure grammar." I have no doubt about what you said, however, there can be added something in behalf of the grammatical element of the issue. Being a non-native speaker, I wouldn't say it that way too, but these two are just examples to show and find out how different forms of the infinitive work.
Do you find the interpretations of what each sentence means to be correct?
1) You haven't provided any info saying that his parents/relatives are not English-speaking people.
2) He must have had his script proofread by a native.
3) May be you think that Otto Jespersen's work should also be discredited on the ground of him being danish?
I have closed this thread - it seems to be getting nowhere fast.