This is going to be a two-part question.
1.It's been a long time since I've seen him.
It's been a long time since I last saw him. (or ..since the last time I saw him)
Do the above sentences mean the same thing?
It'd been a long time since I last saw him.
It'd been a long time since I'd last seen him.
Do both the sentences, again, mean the same thing?
When we're using the past perfect, do we keep the entire thing in harmony, or could move the next tense to the simple past? Like, "I hadn't seen him since we were in school together.", or should it "I hadn't seen him since we'd been in school."