Gillian foreshadowing only really happens for the player if they have Lena fuck Mike, and even then Ian is not privy to any of that info, so it'd still be a big shock for him specifically.
Mike fucking Lena or Ivy is less of a thing because these too aren't things Ian is privy to (even if Jeremy reveals Lena's cheating, he only says "she's been fooling around at the club" which covers Mike, Mark, Marcel and Jack) and i think neither Lena nor Mike would reveal their relationship in front of Ian unless he's on cuck path.
So, in terms of drama Gillian would be the only near-guaranteed point, though of course so far it's still nothing but player conjecture and maybe we'll just all get disappointed. :v