Question for you before you go any further.
Have you tried to operate the wheelchair lift. Extend and retract it?
When it is extended and in a safe position with no power, have a look at what bolts there are for mounting the whole unit. It might be wise to take a good look at what you are up against before disconnecting the unit.
You can see if any switches will interfere with your ignition if you get into trouble later starting. Take both cables off and make sure they can't splash on any grounds while you work.
The headlight question you have. No, they do not really need to have the battery disconnected to repair. Just pull the connector off carefully. You may or may not have to change it also if it's corroded after being in service for years.