As a DIYER that has paid for the simple service, a simple hub license and a simple multihome license and 100 devices…
Should I be using Simple Home app or Simple System app?
And as far as my guests go do each of them have to pay the $10 app fee in order to be able to control my properties?
Or is there a free version that they can use that allows them to get guest permissions from me?
Home and System are effectively the same app. The difference is that buying Home for $10 gives you 3 extra devices that are not tied to a subscription. Both of them will be getting all updates going forward.
Guests can certainly use the System app for free assuming your configuration doesn’t relay on those 3 devices that Home gives you. While given guest permission by you to use your configuration, they are using your Simple Service so they don’t need to purchase anything.