I am in the same boat...
Tried installing x3, got that same api missing .dll message. Saw the forums and tried reinstalling 2015 redistrutable, no luck, firewall and avs off no difference, tried downloading automatic updater and letting that do its work I got error messages. My computer has been offline since 2012, Ideally a clean install would be nice, but with a large library of plugs and and everything else functioning properly, doesn't seem this will be a option. Can you point me to offline updates from microsoft? or possibly help me narrow down files needed?