I think you get the basic idea, but you aren't quite sure yet about when to use this verb tense.
For the first two sentences you wrote, I'd write them more naturally as:
1) The teacher hadn't assigned any homework, so I had some free time on the weekend.
[This is appropriate if you are telling a story set entirely in the past tense and you are, within that story, telling about something even further in the past. Having said that, it sounds a bit literary to me because in North America at least in conversation, we'd often use the simple past in this case: "The teacher didn't assign any homework..."]
2) There weren't any tasks left, so I was able to leave work early yesterday.
[I'm having a hard time imagining using the past perfect in this case. Maybe it's because "no tasks left" and "able to leave" are happening at roughly the same time. You could say something like, "Because I hadn't been left any tasks to complete, I was able to leave early." It's almost the same, except that the action of leaving tasks happened (or didn't happen) previous to the person leaving work.]
[A note about "could" compared to "was able to": Theoretically these are interchangeable, however, "could" has the alternate meaning of future possibilities ("I could go camping next summer".) For that reason, when we are expressing things that were possible to do in the past, it's usually better and sounds more natural to say "was able to" rather than "could". On the other hand, if something was not possible to do in the past, it's perfectly natural to use "couldn't". Also, if you are reporting that someone said something like, "You can leave", it's natural to say, "He said I could leave."]
3) I hadn't been feeling comfortable there, so I might have seemed absent.
[This is fine grammatically except you need to add "have" in the second clause. Also, "seemed absent" is not an expression I've often seen. Perhaps "seemed distant" would express your idea.]