I just finished a particularly tough and close game against Zhar using the Kingsport Expansion. At some point the Mythic Environment 'Worlds Drifting Apart' appeared, granting a merciful respite. While it was active the Mythos card 'Unusual Phenomenon Sighted in Arkham!' came into play.

I guessed, that since the Environment prevents rift markers from being placed, I just got lucky.

A couple of turns later, the rumour 'World Torn Asunder' came into play. Incredibly the Environment was still in play when another three turns later, the rumour failed. Following my previous reasoning the Environment should have prevented anything bad from happening but that seemed a bit cheap to me.

Does the mythical proto FAQ have anything to say on the interaction of these Mythos cards?

I don't know if the FAQ says anything on it, but I'd say you played it right. Preventative effects take precedence. You had to open four rifts, but rifts can't open. Similarly, if you fail the Terrible Experiment but "Happy Days" is in play, you have to raise the Terror Level to 10—except that the terror level can't increase.

You lucked out, big time.

Thanks Tibs! Your confirmation of my reasoning is almost as good as a quote from the FAQ

Tibs said:

I agree with Tibs. Environments take precedence over other game effects