My group of 4 had recently started playing SeaFall and just finished prologue last weekend.
The gameplay was decent, but we find a title card, the Prince/Princess missing.
Would that affect our future gameplay?
Could we replace it with the remaining title card we haven't used?
Or is there any way to make a replacement order?
The only real purpose of the title card is to record and establish the turn order for the next game. You can probably look at the title cards you have and reconstruct the missing card, or do as you suggest and use the remaining four, skipping the Prince card. Or you could re-label last card (Lord/Lady) as Prince/Princess.
The most important things to remember regarding the title cards are:
* They are determined by campaign glory (starting at the end of Game 1).
* They determine seating order (and turn order). Make sure that as you go around the table clockwise, you have the lowest title, then the next lowest, and so on up to the highest title.
* They determine tie breakers. Ties are always broken in favor of the lowest title.
As long as you have those straight, you can play the game without the title cards.
And to Ben, that's not necessarily a Game 1 spoiler -- it's a Box 1 spoiler.