Weird. I'm not sure how this would work in the rules, but in principle you should be able to see what gets cast off the cascade trigger. I guess you could just define "When ~ cascades into a * spell" to mean "When you cast a * spell exiled with ~'s cascade ability." My only suggestion is that you make it a may ability, in the off chance you really want it resolve anyway.

My goal is that if they cascade into (I.E. reveal any dragon that they could cast from the cascade ability ) that Thunderfall Dragon goes back to the players hand whether or not they choose to cast the revealed dragon. So if I cast this and reveal say Dragon Whelp then thunderfall Dragon would go back to my hand reguardless of if I chose to cast the welp.

Oh. If that's intentional, go for it. I wasn't sure.

Yes it was intentional. I just wanted to make sure that the wording I used works as intended.