Flight 714: Hergé's plan for the aliens?

Alright, so I've searched this a bit and I couldn't find anything much pertaining to this specific question. There's a lot of information skirting the edges, but not this exactly... I think.

As we all know, in the end of Flight 714 all of the main characters are taken up into the (controversial!) flying saucer.
I've read (mainly in books like Michael Farr's Tintin: The Complete Companion,) that Hergé was criticized for using such a metaphysical component so prominently in the storyline.
I've also read that even Hergé himself voiced some retrospective regrets on the use of this element for the book. (I think he mainly said something about showing the UFO "too clearly," or something like that.)

Tintin's party show up again with their memory wiped; Rastapopoulos and company are never mentioned again.

Skip forward to the unfinished Tintin and Alph-Art where we meet the character Endaddine Akass. Some people seem to believe that Akass was merely another manifestation of Rastapopoulos, others have different candidates from the Tintin books.

If we assume that Rastapopoulos is indeed Akass - a fairly common belief - that would mean that Hergé had at least come to grips with whatever happened to Rastapopoulos in the flying saucer.

Is there any more detailed information than what Michael Farr mentions in his book out there that anyone knows of concerning what Hergé thought about Rastapopoulos's abduction?

Did Hergé ever speculate on what might have happened to Rastapopoulos and Allan in the UFO?

If Dr. Krospell's fate is anything to go by, then Rastapopoulos and the others would also have ended up somewhere on Earth and without memories, hence his speculative re-emergence in Alph-Art. Take a look here for other theories about the abductees.

Huh. You know what, I completely forgot about Dr. Krospell turning back up in Cairo at the end of the book!

I guess Hergé already created a kind of 'out,' then. Whenever he wanted Rastapopoulos to turn back up all he had to do was have him come back with a story of a memory lapse, like Krospell.

Good answer.