Once you successfully import a build, it should appear as an option when you click "Add a version". Do this, then click "Update Menu Options". This should make the new version appear in the app manager. It looks like there's a caching issue there, but if you navigate to the edit builds menu page again, it should reset the cache.
HHH10000002: File or directory named by URL [file:/home/cchq/www/monolith/releases/2019-09-13_07.44/formplayer_build/formplayer__2019-09-13_09.52/libs/formplayer.jar!/BOOT-INF/classes] could not be found. URL will be ignored
For reference, I don't believe this error is what's causing the formplayer compile issue as I have seen it on our QA server which has a month old build on it which is compiling forms OK.
These files are managed automatically by commcare-cloud (see here), but you can inspect them on your server at /home/cchq/www/monolith/releases/current/localsettings.py
and /home/cchq/www/monolith/releases/current/formplayer_build/application.properties
That sounds likely - commcare-cloud manages those files, so you can't modify them manually. The formplayer port defaults to 8181, and you may be able to override that in your environment's public.yml, though I'm not certain.
Confirming what Ethan said. If you set the value of formplayer_port in public.yml and run the cchq <env> update-config command referenced above, followed by restarting formplayer, that should do it. For example to change the port to 8182, you might add