To add on to the advice you have already seen, since many rental scooters appear identical, you can bring along something bright and colorful - like a bandana, or a length of ribbon, or even a Disney shirt, or a beach towel - and leave that on the scooter to “mark” it. That way, if the scooter does get moved while y’all are on a ride or in an attraction, you can ask the nearest CM if they have seen the scooter with the bright blue polka dot ribbon tied on the basket.
Y’all need to call the scooter vendor that you are most interested in using, and talk to the them. Part of the discussion will be around your hubby’s height/weight to make sure that he is set up with the scooter that is the best size for him. If he has (for example) any arthritis in his hands, this is when y’all could ask for a scooter with thumb controls instead of a squeeze throttle.
Last but not least, yes - you can actually go online and order a generic key that will fit at least 30% of the rentals at Disney on any given day. Rental vendors in Orlando buy huge fleets at a time, and the cost to key each one differently would be prohibitive.
Be sure to ask your rental vendor what their policy and procedure is if there is a problem after business hours.
And be sure to park the scooter *inside* your hotel room every.single.night to recharge it overnight. It’s important to keep it in your hotel room. There is ample space in every single resort hotel room at WDW for a rental scooter. If you need furniture moved or rearranged, just call Housekeeping from your room phone.