Earlier quoted context omitted.
I usually fix 3. by redirecting the intermediate program to stderr before piping to pv. My main use-case is netcat (nc). As an aside, I prefer the BSD version, which I find is superior (IPv6 support, SOCKS, etc). "GNU Netcat" isn't even part of the GNU project, AFAIK. I also discovered Ncat while writing this, from the Nmap project; I'll give it a try.
I don't quite understand what you mean — by default, most Unix-pipeline-y tools that produce on stdout, if they log at all, already write their logs to stderr (that being why stderr exists); and pv(1) already also writes to stderr (as if it wrote its progress to stdout, you wouldn't be able to use it in a pipe!) But pv(1) is just blindly attempting to emit "\r[progress bar ASCII-art]\n" (plus a few regular lines) to…
For a contrieved (but the first I can think of)
dd if=/dev/abc status=progress 3>/dev/null | pv img