Possibly related to all of this download failed / checksum failed madness, there's been several times where I've tried to download something with wget on the command line over HTTPS and have it bomb out (it was an IPK I was trying to grab). Ended up just SCPing the file over from my PC directly to OpenWrt and then install and sure enough, worked great.
I also had all of the pre-reqs installed for HTTPS such as the ca-certificates and ca-bundles at the time.. and it let me download it, but the size of the file did not entirely line up with the file I copied over locally with SCP and by association the checksum was also wrong (file was corrupted).
I don't think this is a problem with your build in particular, but something with OpenWrt and HTTPS is my theory. Thought I'd throw my $0.02 in here with my experiences so far, and best of luck nailing this annoying bug down.