So my rough theory on the reaper quest and why Emet let him go is he was seeing what could come of the connection to the void. Since Void got messed up and is currently in a non-rejoin state he may have wanted to see if they through the reaper pact could pull out the void and rejoin that way. And that rough idea is why I think Emet let him go.
That's one possibility. Since we get nothing about it from Emet-Selch, nor did Drusilla know what happened to Rullus that night, we can only speculate.
I'm certain Emet-Selch must have had an idea about Reapers, given their role in Garlean society for so long, and his ability to learn secrets. He might have found them a curiosity, an experiment, something to let simmer to see how it exploded later (a LOT of plans, from Allag to the modern era, by the Ascians are time bombs left to wait for an advantageous moment to go off), maybe he was bored that day, feeling magnanimous, amused by Rullus' attempt and reaction...who knows?
The connection to the void though, and the possibility of salvaging something from that mess, would make for a handy argument. I gather it took some time before Orcus was loosed, though, from what little we know.
My preference for fanfic writing are these missing scenes and filling in the gaps, speculating on lore-compliant ideas, bandying them back and forth, and seeing what sticks. There's a lot of space within the lines to play in and fill out.
