I must have good timing, just caught your post! 
1.) dfu-util link for others for windows is here: http://dfu-util.gnumonks.org/releases/dfu-util-0.7-binaries.7z (in the win32 folder)
2.) – yeah, installing the drivers on windows isn’t great yet, working on it!
4.a.) dfu-util will assume you want everything when you ‘upload’ from your flash to your computer. so you can append a size with something like: “dfu-util -d 1d50:607f … -s address:bytes -U destination” – This’ll be wrapped in the command line tool.
The der file is just a binary format of a public or private key, but you can grab that file here:
https://s3.amazonaws.com/spark-website/cloud_public.der
Looking forward to adding your new public key, Thanks!
David