You will love this trip! It is fantastic. In BA 3 days makes sense. There is a lot to see and do. Great restaurants too. I have been a number of times and I think 3 days very worthwhile. Santiago one day ok unless you want to do wine tasting, etc. I highly recommend the expensive trip to Torres Del Paine. We did it and we're very happy we did. Sign up soon as it will sell out as only a limited number of people can go. The weather could be iffy, but we lucked out and has a nice day. Beautiful scenery.
If you can I suggest for the way you are going a room on the starboard side would be good for going through the Chilean fjords. If you are on the other side no problem but for that one part of the trip it makes sense. We went the other way and we're on the port side and did not have to leave our room to see many things but it's only for a short time.
Antarctica itself was really amazing. We lucked out on the weather. Seabourn dies an amazing job organizationally. Only 80'people can go ashore at once. You have a colored wristband and go at different times each day. It ran like a clock. They help you get into your boots, into the zodiacs, have a fantastic expedition team that goes ashore ahead of time and maos everything out for you. It is first class all the way. Lectures are every day but you can go in person, watch from the tv in your room, or they record them so you can watch on your own time. Be sure to rent the boots. If your boots don't fit they have a " boot exchange" and things worked out for everyone. We also rented the walking sticks. Some days they were helpful, but some nit necessary. Good to have depending on the depth of the snow.
We bought most of the recommended clothes, but fir us the weather was warmer than expected. Not warm, but nit as cold as exoected is probably a better way to describe! If you are taking iphòne photos get glove liners that work with iPhones. Look online as otherwise you have to take the gloves and liners off. Especially if you have a screen cover protecting the glass. I also had a thing to hook it around my neck so did not have to worry about dropping. My husband carried the serious camera gear. Drakes Passage was rough but not as bad as I expected. Once in Antarctica it was very calm and well protected. The crew and expedition staff go out of their way to make it a great trip. All expeditions in Antarctica are included except kayaking which I suggest you try once or twice if so inclined. It also sells out so plan in advance. Ifctgecweather bad they will cancel. MDR was open for breakfast and lunch every day on our cruise. But they had heaters and blankets on the pool deck so lots of people ate outside in Antarctica too. Feel free to PM me if you have specific questions. I'm happy to answer!