The Witcher 3: Hearts of Stone game doesn't have many endings - the main storyline ends always in the same place, where Olgierd von Everec or Gaunter O'Dimm will say goodbye to this world. Despite that, you can select whose side are you on...
To have a full choice (the possibility to save Olgierd), you must complete the side objective in the last quest Whatsoever a Man Soweth..., which is meeting with Shani and then with professor who will tell you how to defeat the Master Mirror. The romance between Geralt and Shani doesn't affect this objective. The choice from the Scenes From a Marriage doesn't affect the ending as well, but occurs in it. Depending on whether you take the rose from Iris or you take the canvas part while refusing to take the flower, the item you taken will be given to Olgierd.
If you haven't completed the side quest or you picked O'Dimm's side, you will receive a reward of your choosing from him (and some experience):
- To be rich - you will receive 5000 crowns,
- To never feel hungry - you will receive the Horn of Plenty which you can use as an infinite source of food,
- To possess a bottle of liquor that is never empty - you will receive the Bottomless Carafe. Sadly, it has no actual value, you cannot dismantle it or use it,
- To be fast as wind - you will receive the Caparison of Lament which gives 100 durability points for your horse,
- Refuse taking any reward - you will receive more experience.
If you choose to save Olgierd, the failure will end in death of both of you. However, if you succeed, you will receive the Iris' saber from him. During the additional level to which you will be transported to solve the clue, you can also find the Viper venomous silver sword. More details can be found in the chapter about the Whatsoever a Man Soweth... quest.