Update:
After spending 160 gems last night to speed up the final battle, then spending time yesterday evening, overnight, and this morning trying to earn as many of Hans' items as possible, I was able to spend 154 gems to buy the remaining Hans ears (I earned 3 and needed 5 more). With 3 minutes left in the event I was able to welcome Hans to my kingdom. In the last 10 minutes, I had Sully, Mike, and Woody/Bo Peep on quests that might have dropped ears, and none of them came through. Glad that I had enough gems! I'd already spent the $$ to purchase Olaf & Sven, and didn't really want to sink anymore money into the game at this time.