The embroidered blocks are absolutely adorable
I tend to agree that the sashing is a bit busy for the embroidered blocks.
IMHO I think adding cornerstones to the snowmen sashing would be worse than it is now.
At the same time, I don't think the original sashing fabric looks totally out of place.
Somethings to consider ...
Will the original donor be disappointed or even insulted, if the snowmen fabric is still not there?
Will they recognize and be upset that the blocks and sashing have been taken apart?
Will there be bedlam around the church because you changed it?
I have no idea as to your answers or whether they even matter.
Howeverrrrrrrr ... I also know how things can go around churches!
Is there an end purpose for this quilt?
i.e. a fundraiser? gift to someone at Christmas? selling at a bazaar?
Or perhaps it is yours to keep and do as you wish ... and the thoughts/feelings of others do not enter into this.
The answers to these may help you decide how much work you really want to put into this project.
My thoughts would be to either keep with the original sashing, or switch it up to a blender-nothing-print in a rich blue or deep cranberry red. You may even be lucky enough to find a small snowflake print that could amp the adorability even more while not taking away from the amazing embroidery work.