Hi Cristina! I'm so excited to see you will be at Walt Disney World Resort in a few short days!
I love how thorough you are being with your planning! I know rain or shine, your vacation will be magical!
I would definitely pass on the umbrellas. They get bulky and cumbersome as you are walking through the theme parks. Ponchos or anoraks are the way to go. In fact, this is one area where we will splurge a bit. We typically purchase some fairly high quality rain gear prior to our vacation and take it with us. As a bonus, the rain gear serves as a coat inside attractions if the air conditioning feels cold after being out in the warm and wet Florida weather. In the past, we purchased a Disney Parks poncho and used it to cover our backpack when we placed it on our stroller. It worked beautifully.
Shoes are a personal preference but we prefer tennis shoes over sandals as we have had fewer blisters with tennis shoes. An added step might be to pack some blister relief pads. They are small and portable so having a few with you in the theme parks may be helpful. If possible, I would try to take 2 pairs of shoes per person as well as a few pages of newspaper. When you return to your Resort hotel room for the evening, place newspaper in each wet shoe to help it dry a bit faster.
When you are mapping out your itinerary for the day, you might want to select a few attractions or entertainment with indoor venues. We have spent hours inside the shops on Main Street, U.S.A, or in
The Seas with Nemo & Friends and loved every minute.
It sounds like you are going to going to make the most of every moment rain or shine! Some of my family's most fun and memorable Walt Disney World Resort memories were made on wet rainy days. Enjoy every magical moment!
Kari