Yeah the sweetener is completely optional. I imagine they probably use TFA-sweetener which is a combination of Ethyl Maltol + Surcrose. or they use supersweet and add the Maltol separately.
I whipped it up and its pretty solid. I followed your recipe to the tee. Im gonna let it sit for a bit too. Made a 10ml tester. So far on the first 30 hits, its solid. Great recipe!
It didn't change much for me with steep. To be honest it tasted nothing like propaganda's to me. I have no idea what they use. I can't pick out a single flavor that might be in it. I've had ideas, and ordered a bunch of concentrates chasing it. I just ran out and need to get more so I can keep perusing. I'm guessing it'sprobably just two main flavors combined that gives Blue Slushee it's unique flavor. Every time I think I'm on to something, it's wrong.