I don't have the book on me at the moment, but when he and Tintin come face-to-face, I seem to recall that he boasts about being "alive and well" as if he'd again faked his own death in their previous encounter - maybe "Red Sea Sharks".
I've always doubted that the aliens actually took Rastapopoulos and his associates to another planet in "Flight 714". It does look like Herge intended to use him again in "Alph-Art", we'll never know for sure. But how about this:
Like Krollspell, he is dropped off back on Earth but, being of a more tougher and forceful nature, he recovers his memories and starts stealing valuables museum exhibits from his underwater base - which he set up while he was the Marquis di Gorgonzola.
Rastapopoulos is a very active man. In the early books he is both a respected film producer and the leader of highly organised drug smugglers. In "Red Sea Sharks", he's involved in smuggling weapons, slaves and overthrowing foreign regimes. It figures that he has more than one enterprise, crooked or otherwise, running at the same time.
why does he even need the Professor's machine in the first place
Sheer greed. He's been stealing valuable jewels and works of art and replacing them with fakes, but even fakes are expensive to create so Calculus' machine would save a lot of time and money.