To eliminate all variables, I just did a blank install of Joomla 3.8.11. Attempting to install jsn_teki2_1.0.1_install.zip yields:
Warning: count(): Parameter must be an array or an object that implements Countable in /data/sites/http://contoso.com/tmp/install_5b7a1b53e024f/installer.php on line 420
ERROR: syntax error at or near "LIMIT" LINE 1: ...rotected=1, ordering=9999 WHERE element LIKE 'sunfw' LIMIT 1 ^
I've tried a lot of different solutions - using discovery, installing sunfw separately, hacking installer.php to not using a limit clause on an update - it all seems to end in the public site displaying this message:
Error: Class 'SunFwSite' not found: Class 'SunFwSite' not found
At a few points I've had it where sunfw and the template were both installed, but I always end up back at that same error.