The former chairman of the pathology dept. where I trained had his international dermpath boards. And he was an AMG who was AP/CP board-certified who completed path residency in the U.S. in the mid-seventies and did a clinical chemistry fellowship (BC'd as well) when it was still hot, but later wanted to become more proficient in derm. He already had been signing out derm for years so I don't know what additional benefit having int'l boards would have been because the group already had a dermatopathologist. And this was at a U.S. teaching hospital that was JCAHO, CAP, ACGME, etc. accredited with all BC'd pathologists.
To answer your first question if there's any benefit if one has ABP dermpath boards to then obtain int'l boards, probably not because ABP boards are pretty much universally recognized. As far as what the qualifications are to sit for them, I don't know. Why somone would pursue these boards could also be a matter of personal merit/achievement. I think one benefit is advertising for someone who did not do a dermpath fellowship here and is already in private practice, the group can say that they have a BC'd dermpath guy. After all, they did pass their dermpath boards, it was just written/issued by a different governing body than the ABP.
Now if actual ABP dermpath boards (and therefore a U.S. fellowship) are required to get hired for a job that's seeking a dermatopathologist here, the employers will most likely require that.