You may have trouble doing that if you're trying to use IFPC from a frame within a frame.
Nested Iframing like that also may be against our terms, if i recall correctly.
In any case, it's probably not the best way to go. The external iframe approach is not legacy - "Legacy 0.8 App Editor/External IFrame Apps" meant that you'd need to use the legacy 0.8 editor to use external iframes. You might want to try that approach.