That's a darling slipper. The pair will be 2x cute!
Is it possible you could use a provisional cast on, then BO those sts as well as the last ones you knit to make it match? There are various stretchier BOs that you might look at also. I'm sure we can come up with links if you wish.
I've been following this thread but as I've not done baby shoes or slippers or booties, I wasn't sure what's what with the pattern. The not stretchy enough BO I'm familiar with.