Dear Eckard,
I have in fact proven the possibility of this third alternative (the one you state, not Pentcho: I'm for the two postulates, just not the third---that absolute space and time are superfluous and can't be detected---and I DO NOT sing Divine Einstein) in my essay. It is by definition impossible for "observer" C' to interact in any way with "observers" A, B, and C, who exist in an absolute "flowing" present.
By the way, with regard to your Saulus/Paulus comment, have you by chance read Ref. [9] by Weyl in my essay. It's quite a neat article. For example, after Petrus opens with "Lass uns heute ausfuehrlich darueber sprechen, warum du nicht mehr glaubst, dass (*M*) die Traegheit eines Koerpers durch das Zusammenwirken aller Massen des Universums zustande kommt. O Saulus! Saulus! wie kannst du dich so gegen die offen zutage liegende Wahrheit verstocken!" Paulus responds by saying "ich deine eben ausgesprochene Ueberzeugung nicht mehr zu teilen vermag; und wenn hier der Fels liegt, auf dem die Relativitaetskirche steht, o Petrus!, so bin ich in der Tat ein Abtruenniger geworden. Aber um dich ueber meine Ketzerei ein wenig zu beruhigen..."
You can call me Paulus if you'd like, because I do think space-time geometry is Lorentzian, and that the mathematical theory of relativity is correct---just so long as you know that I'm for a presentist interpretation of the theory that involves an absolute cosmic time and is therefore fundamentally at odds with much of what relativity theory is commonly supposed to describe.
Daryl