Can a current 401(k) elect to start the EACA or QACA arrangement mid-year, provided the Notice is timely distributed?

According to the propsoed regs, the answer is No.

A QACA follows the same adoption rules as a traditional safe harbor plan, and an existing 401(k) cannot adopt a safe harbor except for the entire plan year. An EACA is defined as meeting certain requirements "for the plan year", which probably means that it must be in place for an entire plan year, regardless of the safe harbor rules which might allow an adoption for less than a year to an existing PS plan (because, remember, an EACA does not have to be part of a safe harbor 401(k) plan and therefore does not follow safe harbor 401(k) rules).