“In the midst of so much enjoyment, would the non-Ariya devas simply spend their entire existence enjoying sense pleasures and neglect to cultivate the path?”
Yes. Most of them would not even think about samsaric suffering, since they see very little suffering. We can get an idea about a rich, powerful, and healthy human. Would he/she have time to even think about such things. They will be busy with enjoying sense pleasures and making more money.
“Even given the fact that they may have accumulated infinite merits in their infinite existences,..”
No living being has an infinite existence. Their lifetimes may be very long in terms of our time scale that we are used to. However, even the lifetime of an arupavacra brahma (which can be many aeons), is a just a blip the samsaric time scale.
“Even in the human realm, we can imagine that some rich and powerful humans believe that suffering is only for the disadvantaged poor. They do not realize how much suffering there is, until they are severely old or sick and it is too late.
Quite true.
“I am fascinated that devas and even brahmas would believe in the merits of learning and practising Dhamma. It does not seem very likely to me that they would think so..”
There are many devas/brahmas with lower stages of magga phala, below the Arahant stage. They are certainly interested in listening to Dhamma, if it is correct Dhamma (They will know the difference too). Furthermore, there could be many who are Sotapanna Anugami, and would be very much interested. They also discuss among themselves.
“Do all devas and brahmas have supernatural powers?”
The term “supernormal powers” is again relative to our (human) capabilities. Each bhava has its own capabilities and drawbacks. For example, owls can see at night, but humans cannot. That could be called a supernormal power for an owl. How do some birds fly many thousands of miles during seasonal migrations? That is a “built-in” capability for them that humans do not have. There are many examples like that.
-The ability to travel long distances in short times is another characteristic of deva and brahma bhava. Not only them, but also human gandhabbas can travel at high speeds too (they have fine bodies that allow that); however, they cannot travel to deva/brahma realms, they can travel only within the human realm.
Again, we can only speculate about some of these things. But we do know that devas/brahams came to see the Buddha, because many such incidents are described in the Tipitaka.