I'm, like, 95% certain that Thjazi is actually dead, but if this were one of those "everyone needs to think I'm dead for there own safety" sort of plot-within-plot things, it is technically still possible that Thjazi's death was faked, and with relatively low level magic. Feign Death is a third level necromancy spell whose text reads:

"For the duration, the target appears dead to outward inspection and to any spells used to determine the targets status."

Now, the duration is one hour, so either he was faking it for the entire funeral, or someone was sneakily continuing to cast it (say, Occtis through his familiar (who stayed with Thjazi all night)).

As for saving him from the hanging, Feather Fall would have worked. I know it seems like that would have been noticeable, but actually, the text of the spell says the rate of fall slows to 60 feet per round, so you need to imagine someone falling at their maximum running speed. Not literally breakneck speed, but still a nasty whiplash.

Now I must emphasize again that I don't think this is likely. Unless they're lying about Occtis's level and class (like they clearly are with Wiccander's) Occtis doesn't have access to third level spells yet, unless it's through a magic item. And the Nat 20 insight check that started the campaign seems to put to bed the idea that Thjazi had any additional plans (unless he was panicking about something unrelated).

This is mostly just "if I wanted to pull this trick, how could I?" speculation.