Powdered Jello gelatin is about 85% sugar and 10% gelatin by weight. Weight of flavour/colour 5%. Primary function in cake batter is like sugar, and flavour/color come along for the ride.
Instant pudding mix has sugar and modified starch, harder to give proportions. Starch has been cooked and freeze dried to reconstitute into a pudding "instantly" and it grabs water as soon as liquid is added. It holds the moisture better in baked cake. It also decreases the effective gluten content causing a more tender cake crumb.
NO they aren't interchangeable. The cakes will be different.
It could depend on the recipe. If it's a scratch recipe that calls for pudding, then no for reasons stated above. However if you are adding it to a mix, then I have interchanged them successfully in the past to have different flavor combinations that I could not get with the puddings. I'm sure this is because the cake mixes have so much stuff already in there that the stability is not effected, but I don't really know the science behind it for sure.
When life gives you lemons, make grape juice and let everyone figure out how you did it!
I have interchanged them in doctored mix recipes and had wonderful results. I was converting a lemon cake to key lime. Since there is no key lime pudding, I used lime jello, It was wonderful! Texture was almost the same as the original recipe and the flavor was amazing.