Great questions. They’re quite common. Having an intermediate goal does not need to derail your preparation for the main goal. What you need to be sure is that you are well prepared for the ski trip in 5 months so that you do not drain you bank account too badly during it and require a month to recover.
Because I do not know your current fitness level I can’t give detailed suggestions but I can offer some general guidelines.
The first 5 months of the 24 week expedition plan are only laying a base for the harder work to come later. If you are at a fairly low level of fitness then you definitely should do those 5 month of this 6 month plan. If you are at a higher level of fitness (maybe you can manage no 8-10 hours of training a week and have a decent strength background. The I would jump to week 9 of the 24 week plan now and then after a short rest from the ski trip do the final months of the 24 week plan.