To my vast surprise, it seems the event will go on for another week... which I only found out tonight. As a result, I can take things a little more leisurely... and I might actually get enough copies of Rita to mincost her. I got a lot since it took me quite a few tries to 3-star that particular stage... and while I didn't actually reduce her cost or improve her skill any (13 and 9/10, respectively), after combining each copy with 3 Platinum Spirits, she's now the first Platinum unit I have that I've Awakened and fully leveled up. I somehow managed to Awaken and max level Katie, but no other unit.