If I don't have the park hopper can I leave a park, go to a lunch at a resort, then come back to that same park afterwards

Yes, for sure!

Park hopper means you say, Go to Epcot in the morning, and then decide to do to Magic Kingdom later. You can also go to a resort in between.

Yes you can!

Yes you can all you need to do is have your park ticket with you

Yes

Thank you sir

Yes , but it's better to stay on location.

Yes. Same park only though.