Your original phrasing
senior executives, decision makers and policy makers
is correct. If it's desired to state "makers" only once, then "decision and policy" is a list of two items which needs an "and" before its last item; thus "decision and policy makers". This in turn is the second item in a list of two; this list needs an "and" before its last item. So we get
senior executives and decision and policy makers
Both "ands" are needed. If you'd written
senior executives, decision and policy makers
then the reader would have parsed the "and" as part of the item "decision and policy makers". This results in two items with just a comma between them and no "and". Because there is no "and", the reader would thus expect at least one further item (with an "and" before the last item).