Been a while, but... it eventually deleted. I don't know how or why; the first four tries the sparsebundle didn't get any smaller, it stayed the same size after hours of attempted deletion, my iMac beaming unresponsive. Then all of a sudden it worked. I have no idea why.
What I also learned from Mike at Bombich Software (makers of Carbon Copy Cloner):
"Sparsebundle disk images are made up of multiple 8MB "band" files. If the disk image file is large, it could have thousands of those band files, and it may take quite a long time to delete the file (each filesystem transaction can take a long time for NAS devices, especially if they have a "recycle" folder setting)."
He also gave me tips to delete it more efficiently with Terminal; unfortunately, I got the reply AFTER I was finally able to delete it in Finder.