"Should the PM issue acceptance retrospectively (for any future use) ?" Yes.
"Should the PM point out that the lack of response/acceptance did not cause any delay and even identify the date of completion of that activity." Not necessarily. They could notify as a CE under clause 61.1, but perhaps comment that it had no effect on time and cost so any quotation should be for zero.
Alternatively, they could wait for the C to notify under 61.3 and then, in their response under 61.4, notify that although it is a compensation event, it did not have any effect (bullet 3).