do-release-upgrade questions

[Leroy Tennison]

do-release-upgrade requires the current system have up-to-date patching in order to work. I have a client with a number of Ubuntu 16.04 systems, how long will those packages be available?  If they are going to become unavailable before all Ubuntu 16.04 servers are upgraded, can I download the needed debs and put them on an apt-cacher server (and point to it for updates) until everything is upgraded?
Second question due to my fault, I pressed enter when prompted by molly-guard for the server name at the very end of a do-release-upgrade moving from Ubuntu 16.04 to Ubuntu 18.04 and got in a weird state.  I did a manual reboot and sources.list didn't get updated - still refers to xenial.  I discovered this when Postgresql 10 wasn't available.  The kernel is Ubuntu 18 and a check of package versions shows they are Ubuntu 18.04, can I simply get a copy of sources from an Ubuntu 18.04 system and use it?  What else didn't get done due to an improper reboot?  Is there something I can read to determine this and make the necessary changes manually?
Thanks for your help.
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