Mimer is not that far from EO(not sure if its under 400 miles or not) , but I can see buy offers from Moses End.
Each region has a specific coordinate location that is used to calculate distance between regions. The calculated "center" of Moses End is actually a tiny bit closer to the "center" of Eidulb than the "center" of Mimer is. They probably straddle the distance cut-off line.
And I set up the offers in EO, also there is no way to cancel them once I have set them up.
It doesn't matter where you are when you set up the offer. The offer is based on the location of the your region, not the location of the character when the offer was created.
And you are correct, if you are out of range of your region, then you can no longer control the offer once it is set up. I am not sure if this was intentional, but I believe it was. Annoying, isn't it...