I try not to be overly negative about whatever system setup people choose to use, but it seems to me that Ubuntu breaks audio production more often than other distributions. There are ways to build PulseAudio so it gracefully gives up control when Ardour or jackd requests access to the device. I have never looked into the details of that because on the distribution I use (Fedora) it always just works, I never have to worry about it.
I looked through the configuration files on my machine but did not see anything obviously related to that behavior, so unfortunately I can’t really help figure out if there is a way to make your system behave in the same way.
That sounds backwards, but if you follow the link you referenced you will get it setup correctly. You want to start jackd first, so that jackd controls the ALSA hardware driver, then load the jack-sink and jack-source modules into pulseaudio, which will cause an additional pair of ports to show up in jack that you can route like any other application. Let jackd control the ALSA, then route PulseAudio into jack. I would describe that as routing PulseAudio into jack, since jackd is in control of the hardware settings and final audio routing, and you can start and stop pulse without changing any of your routing settings for audio production.
My apologies, just a slip of the finger I did not notice at the time. Seems like the original poster did confirm that Pulse was not relinguishing control, anyone else know a way to make it play nicely on Ubuntu 18 like it does on other distributions?