Works for me. She's gotta come out sooner or later. Or maybe Weimu's already fused into Reimu and a part of her. Not sure where I was going with this exactly when I put it together.
Three weeks? I can only presume that they started feeding Reimu since it's face appeared, so it decided to just take it easy there since it saw no need to leave.
Like How I theorize that yukkuri only shake themselves off the stalk in the first place because the paste stopped and the flower wilted, indicating to them that they have to eat to take it easy.
Yeah, in this case Weimu either never came all the way out or did, looked around and hopped back in before it closed off. This Reimu's not the strongest willed parent and the other kos kind of encourage it by feeding their cute little sister.
Though if I did this again I'd take out the "three weeks" part. Seems like explaining too much.