IAPs operating in controller-mode cannot "fail-back" to Instant with a virtual controller.
You would have to configure your APs as remote access points in bridge mode so traffic is forwarded locally out the AP instead of back to the controlller. You can then configure your SSIDs to stay up even if communication with the controller is lost.
I'm not sure what you mean by marketplace but if its a large open area outside, you should probably use outdoor APs (AP-275).
If you have not purchased any equipment yet, I would recommend Instant for this scenario based on the information you have provided.
You should also work with your local Aruba or Partner SE.