Headlights are fine, ECU on the fritz, and slow cranking from the starter? Sounds exactly like the main ground cable from the chassis to engine has come undone or is loose. Headlights don't return power through there. The ECU's ground is directly to the engine, which explains the running issue. Your ECU controlled relays will buzz because the ECU can't hold them to ground. The ground cable is the return from the starter, and if forced to use the other tiny grounds, will turn slowly.