So, the USB problem has been solved by download the latest LimeSuite from the myriadrf ppa. In my case, because I’m running Ubuntu 19.04 Disco and the ppa doesn’t support Disco, I had initially installed the version in the Ubuntu repositories, but that is quite old. After modifying the ppa to pull from the bionic distro (see how to here), I’m now running LimeSuite v19.04.0-myriadrf1~bionic. Now my USB 2.0 PC can transmit just fine.
However, I am still running into the strange 16 second delay issue with the GRC flow mentioned above. I’d love to understand the reason for that.