Thank you @sorey and @vegaspatriot,
in the 30 fps RSBN livestream Sorey pointed me to, the first shot was fired at 06:11 pm ET according to the on-screen clock in the lower right corner.
Using kdenlive, this is at 06:42:55 + 2/30 frames (=67 ms) relative to the start of the stream. When going back to the moment when the digital clock transits back to 06:10 pm ET, the relative time is 06:42:21 +25/30 frames (=833 ms) The difference is 33.233 seconds and therfore the time of the first shot is 18:11:33.233 ±0.033 seconds ET. @RawDoku may just add a reference to RSBN.
The 30 fps police cruiser dashcam video 1382_202407131806_Unit5-0.mp4 shows 18:11:31 for the first shot at the status line on top. Using kdenlive again, the first shot rang at 18:11:31 +29/30 frames ( = 966 ms). Hence the cruiser clock was off by -1.267 ± 0.033 seconds
Mills’ Sony alpha-1 clock was actually off by -64 seconds. And this was certainly not due to a claimed network-breakdown or a even a wrongly calculated roundtrip-time, as a certain know-it-all immediatly knew for sure. In context of time synchronization the surname Mills (David L., look it up) should have rung a bell.