You guys are worried about a culture-flip, well i guess you have a reason too be worried, BUT how well does are culture match up with the Portugal's culture??
Correct me if im wrong but if our overall culture is greater, doesn't that decrese the chance of a CF?? and when i say strave them out at Lisbon, i dont mean wait 20-30 turns, 10 should be sufficent to weaken them, and replace any troops needed/lost for the invasion of Lisbon. Lisbon is quite easy to destroy. So here's what id do;
1.get updated intel. on all of their cities, (especialy Lisbon)
2.INVADE i recomend galley's to get the two side cities first.
3.Based off the intel we get, we either:
A.Invade Lisbon
B.Starve them out for a few turns and THEN invade
Once those things are done the Portlgeus (or however you spell it) are GONE, im assuming we want them COMPLETLEY eliminated, which means that we have nothing to gain from them, only to stop them and their TWO wonders.