So many questions, none of which is answered by this frankly insane regulation.
From my brief time working for a logistics company, the rough "replacement" rule was 15 years or 2m miles to look at replacement, though this rule was not hard and fast and older trucks were running. Practically this means a lot of these kinds of trucks will be simply replaced on their schedule with newer electric models.
Whether California's power grid can handle it is an incredibly valid question, since the state seems to do rolling blackouts/brownouts every time there's a fire or an exceptionally hot day.
It actually seems quite weak; a slow requirement for manufacturers to start shipping zero emission trucks, and a requirement for fleet operators to report.