I'll test RBF in Fedora 21 Minimal Qemu & try and fix the problem. I have Fedora-Minimal-armhfp-21-5-sda.raw.xz Is that the same one you are using? My first thought is to try the "--force" option with passwd
I ran into the sample "passwd: Authentication token manipulation error" problem running rootpass.exp in Fedora 21 with the image generated using the latest CentOS packages. Manually running "chroot passwd" also gave the same error. Disabling SELINUX in F21 solved the problem. Can you check if it fixes the problem for you too?
About the first issue you reported; I think I know why you got stuck on the password prompt while generating an image. The behavior seems to be due to delay in arrival of the 'New Password' prompt. I had to increase the timeout in rootpass.exp to 4 and this value might require some tweaking.
The updated rootpass.exp is in the repo now.
Regards Mandar Joshi