Hi,
I'm not sure if calibre can convert the result to epub as the unpacked html might contain mobi specific tags, but you can try to unpack the mobi file with my version of mobiunpack found here:
https://www.mobileread.com/forums/sho...9&postcount=72
As it can handle dictionaries whose source is hundreds of megabytes big, I'm pretty optimistic that it also works for this mobipocket file.
Ciao,
Steffen