(e.g. Linux) - command line or tool is fine. Time Machine backups on local disks are just folders with softlinks, so they are easy to browse.
I highly doubt it. A sparsebundle isn't a file, it's a bundle folder full "band files" representing 8MB (by default) chunks of the disk image. I don't believe Apple has ever published the format. I don't believe the code for this (DiskImages.framework, hdid, hdiutil, Disk Utility.app, etc.) has ever been published as part of Darwin or otherwise.