welp, i'm both happy and irritated at the moment.
turns out my 13" MacBook Pro does not have a dead memory slot after all. it was the ram itself that was faulty. i've owned that ram for years, i never even considered that it could be faulty until i tried it in another computer and it just did not boot. so i put different ram in my Mac and it's totally happy now.
so i need to buy a 16gb kit for it now, but hey that's better than having a faulty machine.
see, completely fine with both slots, booted first try: