[...] "infrarecorder". It proved impossible to remove all the crapware I inadvertently installed (despite considerable care)
Sorry to read your machine has been infected by malware. Yet, I doubt that InfraRecorder is to be blamed.
I just tested it (once again). Went to the Infrarecorder webpage, http://infrarecorder.org/
. Clicked on the Download link in the right hand part of the page. This took me here: Downloads
Decided to download the 64-bit installer version Installer (x64 version, 3.96 MiB)
After about 5 seconds waiting time I was offered to download ir053_x64.msi. Clicked OK.
Scanned the downloaded file with Symantec antivirus. Symantec did not find any reason to warn or complain.
Checked the content of the installation package with the help of 7-zip. Looks pretty much identical to the content of the 64-bit portable ZIP archive.
The setup programme did not offer to install any other software.
No hint that it did do so secretly.
Whatever caused your infection, I do not see any substantial piece of evidence that it was caused by InfraRecorder. There is no hint that either the InfraRecorder webpage or the InfraRecorder installer bring along any kind of malicious payload. And the Mint User Guide did not point you to a website that is more dangerous than any other webpage.
Without speculating, rather because it does happen, and one such incident was reported only a few days ago in this forum:
Perhaps you might like to check your router configuration. In case you find it uses some Chinese DNS servers or any other weird looking DNS servers which do not match your internet provider, then any browser request may have been redirected through some malicious servers without your notice. - Note: I do not pretend this is what has happened. It might. Maybe the root cause for the infection is quite different.