Can the custom message that appears after submission contain some dynamic values from the app submission?

Yes, you can add the variables available in the app while designing the custom message that would be displayed after submission.

Yes. You can go to Edit app -->Configuration -->Other Setting: There you will find the Submission Success Message option where you can type custom message and you can also use any variable from an application in the custom message. Type @ to see all the varibales. Please refer to the below image.