What if the coup was delayed until the consumation of the marriage was possible?
To do that you would have to delay John of Guant's death. When he died Richard II seized the Lancastrian inheritance, which triggered Henry IV returning from exile and eventually claiming the throne. I suppose his death could be delayed for a few years, but its also no realistic that Isabella would return to France without knowing she was pregnant. Henry IV kept her under house arrest for months and attempted to get her to marry his son, the future Henry V. She refused, knowing that her husband was dead. Eventually she was sent back to France. If she's found to be pregnant, they wouldn't let her go.
But to not do so would cause a diplomatic incident with France and could trigger another war, with France taking English possessions on the continent. The Lancasters would be in an awkward position indeed. But, assuming they let her go home and she has a daughter, you better believe that the French would argue that she is the legitimate heiress of England, born a Queen. Depending on what happens on the French domestic front, one could see a reverse Hundred years war, with France claiming the English throne. And as to a husband for the baby Queen, that's difficult.
He would have to be a French royal. Maybe Charles d'Orleans or John d'Angouleme, the sons of Louis I Duc d'Orleans (brother of Charles VI). After that maybe Philip II, Count of Nevers, second son or Charles the bold. Or, if the Valois are feeling especially Habsburg, one of her uncles Louis, John or Charles. I think it depends on what happens in England. If it looks like she can be placed on the throne, then the Anglo-French Queen will marry someone who can be a Consort. If not, then probably whichever uncle is the Dauphin, giving France a direct claim to England.