The 'fix' provided by using the cfg.php tool in admin/cli/ - so credit goes to whomever thought of/wrote/maintains that script.
I now do this on every server:
php cfg.php > allsettings.txt
The hardest part of 'fixing' something in moodle config.php is learning the 'variablename'. If one kinda knows what one is looking for, this works:
php cfg.php |grep additional
Pulls any/all settings that have 'additional' in 'short name'.
Above command displays:
Above from a site that has no additional html inserted.
in config.php file. No reboot nor restart required, just access with browser.
In some cases, such as OP's problem with WSOD, brings site back into view and enables admin to find/edit/change the errant setting.
After the addition to config.php has done it's job, can comment it out by putting // in front of that line, for the next time one messes up!
Once again, it does illustrate that having CLI access to a Moodle server is a very big plus in administering. That's a biggy in choosing where/how to host a Moodle, IMHO!