the minhag makes no sense.. what. so. ever.
so you MAY have a problem of chometz IF teh matza isnt baked well enough…besides for teh fact that it is self contradictory. if you REALLY have a chashash chametz then dont do it on the last day. why should achdus matter…why should it matter if its only mid’rabonon…? chametz is chametz and since when do we disregard halachos mid’rabonon?