Welcome to ThisIsMS, Nicola.
As a nurse, you know that it might be MS… OR it might not be MS. Also as a nurse, you probably maintain your calm and self-control in difficult situations. You know that people around you take their cues from you. I wonder why you must tell people the reason you are not driving; can you nonchalantly say that you have a virus, or the heat, or the bright sunshine or lights is causing temporary vision problems – or your new glasses haven't come in yet – or any other excuse? After all, you don't have a definitive diagnosis.
As for the "leftie" situation… just tell them you are ambidextrous.
You are very understanding of other people's feelings; I commend you for wanting to avoid appearing defensive or rude. There is always the "thank you for your concern; I'm still investigating the situation." All the best to you.