So, Tesla can legally open according to the original county timeline, before Elon’s hissy fit, even though in actuality they’ve been producing cars for Europe and China for a week.
The original timeline was always to reopen certain businesses May 18th, since the first extensions of the lockdowns were to expire on May 15th, and as demonstrated by the entire rest of the state opening up according to that schedule without anyone else throwing a tantrum.
Also, the county doesn't need to do anything to save face. They have nothing to prove. The elected officials can always say they were acting on the information of health experts, and the health experts can always point to the CDC, China, and even the re-emerging outbreaks in Wuhan, Italy, and Germany as reasons for not allowing Tesla to restart operations ahead of schedule and before Tesla had demonstrated they were willing to protect their workers.
As late as Sunday (today/yesterday), Alameda County hedged and did not give an specific date, only saying "early this week" (https://www.mercurynews.com/2020/05/17/sighs-of-relief-and-h...)
On Friday they said sometime next week (did not specify early or not) (https://www.kron4.com/news/contra-costa-alameda-counties-hol...).
May 8th, they said sometime mid-May (https://abc7news.com/when-california-is-going-to-reopen-stag...)
Finally, at the end of April, Alameda County (and the rest of the Bay Area) extended the lockdowns to expire May 31st, not May 15th, as you stated.