Hi @Rick283 I hope you are well!
The most sensible way of going about setting up these new listings is for the property owners to open airbnb accounts and create the listings, or give you their login info to allow you to do it, and then you can set yourself up as the primary co-host if you'll be the main person managing the day-to-day.
If you add the properties to your own account then issues relating to those listings could have a negative effect on your own standing and profile on Airbnb, (you could lose your own Superhost status, get delisted etc from issues relating to someone elses property) and if you ever wanted to stop managing them, there is no way for you to give them back to the owner- you'd have to delete the listings from your account along with reviews etc and the owners would have to start from scratch.
To answer your second question: if you do decide to add the property to your own account and make the owner a co-host, the only calendar they would have access to is that of the property you add them to.