@SeptBabyMom - if you think that by the time the trip rolls around you will be better, then you may have options beyond an ECV.
You could, for example, rent a wheelchair, and use it as a walker when you needed/wanted to stand and walk, but then have it as a seat when you need/want to sit. And with at least 50 teenagers along, you should have no shortage of "helpers" to push you if you need help!
You can also rent a walker with a built-in seat (called a Rollator) from many places. That would let you push the walker (so that you have something to hang on to for stability) and have the seat available for when you need to stop and rest.
I know this may sound harsh, but you will be touring with dancers/athletes who should be able to understand the need to utilize whatever equipment is needed to complete your recovery and rehab. Don't let a herd of teenage girls - most of whom have never had a "bad knee day" in their lives - determine what is best for you.
Use whatever tools you need to make your time at WDW the best it can be. Don't worry about what anyone else may think (or say); make sure that you are doing what *YOU* need to do to complete your recovery.
Finally, please remember this: The people you are with at WDW will take their cues from you. If you act like renting an ECV is no big deal, and you stress that you are using it as a tool - just like people who can't see well use glasses, and people who can't do math use a calculator - as a tool - then the people you are with will follow suit. It will be a non-event.
If, on the other hand, you are obviously upset or anxious or otherwise distressed about it, then expect that the people who are traveling with you will be also.
Regardless of how you decide to handle the situation, I hope you have the best possible trip - and good luck to your group!
Oh - and one last thing. Make sure you have considered how you will deal with airports if you are flying; TSA lines will be brutal this summer (especially departing MCO) and if standing is difficult, or if the long walks through concourses (thinking about DFW where we routinely walk for what seems like hours) are going to cause a problem, you may want to consider how you will move through the airport as well.