Can we use aggregator/active transformation after update strategy transformation

we can use, but the update flag will not be remain.but we can use passive transformation

I guess no update can be placed just before to the target qs per my knowledge

You can use aggregator after update strategy. The problem will be, once you perform the update strategy, say you had flagged some rows to be deleted and you had performed aggregator transformation for all rows, say you are using SUM function, then the deleted rows will be subtracted from this aggregator transformation.

Its a drawback of aggregator , we cant use after update strategy

what will be if the flag will be Insert and SUM in the agggregator T/R?

We can but the results may change what you are expecting in output