As an ECV user myself I'll be happy to answer these questions based off of my experiences.
1- They have a few but yes sometimes there all being used and you will have to wait for one to be available.
2- Yes, you will have to return it to the start of the queue for the next guest to use. This is why you may be waiting. Also, some houses let out very far from the start of the queue meaning you would have to walk extra to return it and get back to any ECV left at the start.
3- If you can bring one that will help but how do you mean mounted? This may cause more trouble than it's worth.