I had a similar question about Greyjoy (Kings of the Sea), but now I'm even more clueless.

In your opinion, if I'm playing a multiplayer game using just the factory 45 cards decks of the Core Set, and a fifth player (or one of the three/four) wants to use Martell, which cards are supposed to be used to keep the game balanced?

If you want to keep the deck list at 45 cards, I'd take out all five of the non-Martell dual house characters (so all of them besides Arys Oakheart) and Mag the Mighty, Denys Mallister, Kingsguard Squire, and the three World Championships cards (Former Champion, A Pinch of Powder, and When I Woke...). That's 11 cards so you'll have 42 still in one of the decks (not counting plots). Then I'd add in a second copy of Dornish Fiefdoms, Water Gardens, and Palace Fountains from the other deck. If you want could use the Narrow Sea instead of one of these.

Yes, it seems pretty rational to me...

Thanks for the suggestion!...