If entering into a marriage under false pretenses or under duress can invalidate the marriage contract creating grounds for annulment, why wouldn't a resignation under duress and the election of new pope under false pretenses invalidate that papacy?
uh huh, exactly
Duress most certainly does invalidate both the resignation and the ensuing papal election. However, all that Pope Benedict said in the period between issuing the resignation and it becoming effective leads one to conclude that he resigned freely.
If he was (and still is) under duress, we need a smoking gun from somewhere else. Failing that, the answer to your question "Is Francis Pope?" is "Yes."