Interesting. Did your IWB score or just assist, I would be curious to know? ,
I see your problem with the WBa, it is related to what De Nile reports about the two defending the same area, but perhaps it would have made it slightly better if you had tried a WB-s instead, creating more space in between the two. In my limited testing, I found that the two players on the flank combined ok as one would attack the inside when he could and the other more to the outside, supporting the IWB in attack, which still does not fix the defensive issue.
Thank you for the warning. I see it as a problem, as well, that the IWB would not press high enough, leaving their fullback to pick apart my defence and my two players marking the same zone with nobody marking the zone higher up. Perhaps this is a risk worth taking though in order to utilise the IWB going forward.