Some really cool ideas here, thanks both

quite a few cards I'm keen to add.
Monarch is great. There are currently 8 cards in the cube that grant it, wouldn't mind pumping those numbers a bit.
Loran and Kibo both definitely deserve slots; I'm always looking for more ways to get artifact/enchantment removal on maindeckable cards and those two do it well. Tataru is also great, and Kami of the Crescent Moon (and by extension, Howling Mine - how'd I forget about that?!) seem like good inclusions too.
I'd previously included Long River's Pull but identified a somewhat awkward play pattern specific to Usurper - there isn't a plausible reason to ever
not gift the card, even when countering a creature spell, because otherwise you're pretty much outing yourself as an Assassin. At one point I was considering a custom version where gifting the card lets you counter target noncreature spell which would fix that problem, but ended up switching to
Familiar's Ruse anyway so stopped thinking about it. Looking through the other gift spells, many run into the same issue, but there are some viable options that I'll look for spaces for:


I suspect Harmless Offering would fall into the trap of never making the final cut of a deck, but Wrong Turn might manage it as a more flexible version of the effect. I reckon it's worth a shot
The Hunted cycle is cool. I worry
Hunted Horror risks being GRBS (play it turn two as Assassin and give a random non-Leader the centaurs, game comes down to a removal check and/or lucking into giving the other Assassin the centaurs) but Hunted Dragon comes down late enough to be reasonable I think.
The collusion cards are an interesting angle that I hadn't considered. Pia's Revolution in particular seems like a slam dunk include! Risk Factor is also tempting.
I'm already running Transforming Flourish, it's probably my favourite of the Demonstrate spells TBH. I do love a
Chaos Warp, why not get three for the price of one?
I might be missing something about Narset's Reversal, is the intention to use it to redirect targeted card draw effects and the like?