I hope this time together and the decisions that you make bring all of you closer together as you jointly open up to each other, and deal with the most difficult of questions. The end result is that a deeper love will be communicated and shared between you all, perhaps more than you have experienced before in life. This can be an extraordinarily painful experience emotionally, but one which is essential to go through, and the results of which are worth the pain.
On a more practical note, when you mention the "3rd option" which is to do nothing as being more pleasant, you need to think this through carefully. A death from a metastasized oral cancer is not pleasant. In comparison to other ways to go, it is nowhere near the top of my wish list of a way to leave this earth. Before you let anyone chose to do nothing, please have one of the doctors explain what is likely to happen, what the physical manifestations of this will be, and to what extent the associated pain can be managed. Besides pain there are other very unpleasant issues that are not as easily managed. I do not wish to elaborate on this here on the board, but you should discuss this option in extreme detail with an appropriate doctor before heading in this direction. This journey will change all of your forever, in ways that in some distant future, you may reflect on as very positive. I know that sounds impossible now, but with the cushion of time between the experience and the end revelations you end up with, this may be the case.