Totally understand ... a 'march' is never fun and full of potential catch 22's. Have learned, the hard way, that even after smooth sailing all the way through version 2's on 4 servers, the test run on a 2.9.x to 3.0.whatever highest at that time showed the error of my prep. A plugin hadn't been keeping pace and I didn't do good enough research on addons/plugins. My 'bad', as they say.
So added to the list of things to check ... plugins/addons ... I knew that ... duh!
So now I have moosh installed on all of them .. run the plugins check to make sure there is a compat version of all addons prior to pulling the trigger on upgrading. Even have to check with 'client' to find out just how significant it is to keep the plugin that hasn't been kept up to date .... 'client' sometimes installs on production to get a look see, then decides not to use (even though there is a sandbox for such things - it's just 'eaiser' for 'client' even if 'client' has been warned to be 'careful' for such play on production).
It's their moodle ... and they can use it/abuse it however they like. Hmmm ... is that why I make the 'big bucks'? (yeah, right).
Anyhoo ... glad you got it sorted.
Kinda shows this is an area that Moodle will one day have to address ... updating and/or upgrading Moodle from within Moodle itself.
That's becoming the 'expectation'.
'spirit of sharing', Ken