The K8 relay is operated by the engine ECU but its the final step of the start process sequence and there are quite a few things that have to happen before the ECU issues the OK to operate the starter. Being able to activate the starter with a hot-wire of K8 would verify the starter motor is working but nothing else in the start sequence of events.
I'm not very familiar with the W163 particulars but I think the ECU gets instructions to operate the starter from the Drive Authorization system. There are more than a few 'moving parts' to the Mercedes DAS system and they particulars change depending on years and versions of DAS used.
https://www.benzworld.org/threads/everything-you-always-wanted-to-know-about-das.1245088/
My cousins issue with S-class was a fuse that provided power to the DAS module - this interrupted the whole validate/start sequence.
Your ML seems to have something called an 'All activity module' denoted as N/10 in the Star Wiring diagrams that provides this function.