Your question is complicated, but the answer can be simple.
If something worked well with rtmpdump, it must work with ffmpeg as well. FFmpeg and Rtmpdump are both based on librtmp.
It is just matter of "translating" rtmpdump command to ffmpeg command.
There is one simple example of that, here.
http://stream-recorder.com/forum/ple...ng-t22585.html
escaping ? with \? appears not to be necessary in this case, so this is error here,
( Required only inside the regex pattern of FINDSTR ).
I noticed it is an error when I saw it showing on command line when you executed the command, on command line
you should see it all "as is", and purpose of escaping in batch file is to see it "as is" on command line.
Appart from that, I cannot see other syntax error in your last ffmpeg command so I suspect parameters themselves.
To solve that I would try to replicate parameters identically from old rtmpdump command that worked.