Your combat formation is based of off which position companions are behind you in the "Formation" tab. Right click your game, get to the player menu, click "Formation", click "change", select a character portrait in the top row(should be the player character and three party members), and then click on a party member's name in the column on the left. That's how you switch out party members. Do that in the order that want, tanks in front, healers in back, etc... note that there are some enchantments that give taunt to certain party members as well.
You'll always want to go for helpful/nice dialogue and player actions when dealing with Carys related content. So sounds like you're off to a good start. Carys has two recruitment quests, the first one is called "Mysterious Note", you'll receive a note from the barkeep in Summeredge. This leads to the start of the quest where you help Carys stop a bunch of slavers. I'm assuming you already did this one. After you complete the quest, head to the area north of summeredge plaza, and up the steps, Carys will come out of hiding and stop the player for a cutscene. This starts a quest "A Cure for Seren", where you have to get a local healer to come and give aid to Carys' blind sister Seren. As the quest is finishing a cutscene will play were Carys' father will come back, and he'll ask the sisters to go with him instead of you. You'll want to make sure that you have above 5 morality. Technically you can have anything above -12 but it's safer if you have above 5 morality. This next part I haven't personally dealt with in depth: I have read that if 2 of your companions each have 5 or more corruption points a flag will trigger. This causes Carys to accept her dad's proposal over yours. If all goes well Carys and Seren will turn their father down and join you at your castle. Afterwards Carys will only leave the player if they drop below -12 morality or take the evil ending to the game. If you need help with getting affection points or morality points check out Redknight's walkthrough.
After completing the quest "Aid for Ornesse" you will have killed Revana. This is important to get a specific item related to the dungeon. Once the quest is completed go to the temple in Ornesse, and click on the red orb in the room. Click it until your prompted to remove a red shard from the orb. If I remember correctly Revana stops the player from collecting the shard. Get the shard, note that it will be located in your quest items tab. Go down to the floor puzzle. Click on the shard and use on the player character. This will highlight the safe places to stand on to complete the floor puzzle.
Try to enter your character's room in the castle. The sex scene should occur after entering your room. If it doesn't occur immediately after the conversation with you Tiah, and Francis, go out and complete some quests/travel. Then come back to your room. Sometimes it takes travel time for the game to update the h-scene triggers in the castle.