Hi, Jennifer! From the time you board the Magical Express bus, you can expect it to take about an hour to get to your resort. It can take several hours before your luggage is delivered to your room, so if you are going to need anything, keep it in your carry on bag.
As for the park being horribly packed, I'm not sure what time of the night you'll be arriving, but I can tell you I have had two entirely different experiences on Memorial Day visits. On some trips, I have found that the later in the day it is on Memorial day, the thinner the crowds get (still very busy, but better later in the day). I imagine that people come in for the long holiday weekend, and take later flights on Monday to get back to work on Tuesday, so they leave the parks early. I will hope that is what happens for you! I have also experienced the parks VERY crowded, even at the end of the day on Memorial day, with people taking advantage of the last minutes of their long holiday weekend. Many southern schools and colleges are already finished classes by then, so some people are just starting their summer vacations.
In any event, I hope you have an absolutely wonderful vacation! I love the Pop Century resort - the theming is so adorable!