Neither of the 1kyuu grammar texts I have show いまさら as required (or even present in any of the example sentences). The 2kyuu one has a meaning (I believe) closer to 'considering A, based on A'.
I am still not clear about this grammar pattern. My teacher (Native Japanese) explained its use as ーたら. eg sentence that she gave is: 新しいワープロを使ってみたところ、とても使いやすかった。 (あることをしたら、その結果がどうなったかを表す表現)
That's a different grammar pattern (no で following the ところ) :) The one you're referring to is a time relation marker, it basically means 'right after [A], [B]' - used to show a following action, or a discovery of info brought about by [A].