So unlike our theories, time does indeed play a factor and 2 years was definitely a good time before Fiona could be a complete vessel.
However I guess that played in the Demon King's favor and now even if Albus restarts, Demon King would know. I think this is definitely where they need to erase him for good because even if they get it out of Fiona, now the Demon King himself is aware of the time loops which is not good for future heroes. I wonder how they'll be able to figure it out in this situation...
Also Albus's look of despair... I'm sure he feels the world crashing down into the bottom pits... Especially with how much he sacrificed for her... ;-;