Hi Laura,
Just like everyone else, I am sorry for your family to have to go through all this. I am an internist (on my lunch break) and has a few advice for you.
If your nana is in sound mind and fully understands what is going on and chose no further treatment, it is in her right to decide that no matter how sad it is for you or your family. In my experience, it seems that she is OK with it and had made peace with the situation. She might even embrace death when the time comes especially if she is religious.
As for your question, no one can tell you how the next 29 days is going to go. She can go downhill very fast or she will be fine for another 6 months. It is great that she is looking forward for the trip and it kept her busy. A positive attitude is always good for any illness. I learn to understand that sometimes situation like this, further treatment might make the end of her journey miserable as compare to being happy until the end. My advice is to concentrate in preparing for the cruise and if something happens in the mean time, deal with it then. The memory of her happily enjoying the preparation for the cruise is just as important as the cruise itself.
One question though, in your post, you stated that you are taking a 3 hours bus instead of flying due to the fear of blood clot. This makes no sense to me at all. Cancer puts people in a high clotting risk to start with and the increase risk of blood clot due to travel has all to do with mobility. If you sit without moving for a long time, it is easier for blood to clot, esp. in her leg veins. Given that, it would make more sense to me that you fly rather than taking the bus. Obviously if your flight is 10 hours long, there is a risk but as long as you get up and walk around every 30 minutes or so, that should be fine. Taking a bus ride for 3 hours sitting there, to me, is much higher risk for blood clot than flying 1 hour.
Lastly, did your nana and you talk to the doctor regarding comfort care? Especially when she will be going on a cruise, you might want to ask and be taught about some Morphine preparation so that if she takes the turn for the worse, you might be able to make her last moment in life more comfortable. This might sound ridiculous but is really not that hard. However, the ship doctor might be able to help in that regards. Having said that, you might want to alert
DCL regarding your nanna's condition so that the ship doctor is prepared.
Hope this help and wish you all the best. Please let me know if further assistance is needed.