I think these are all crucial in the characterisation. Offred is not particularly heroic. What she does is clearly what most of us would have done in her situation. Try to save her family and herself. Having lost her family, trying to save herself. Biding her time, doing whatever she needed to do not to get herself killed or tortured. Always calculating her best odds for survival. Holding onto hope that she would see her daughter and husband again. In dark moments contemplating suicide but not taking any real steps to do it. Finding tiny morsels of good things (mostly memories and fantasies) to keep herself going. Rebelling a little, but keeping it in check. Craving human contact and affection. Letting her guard down when someone shows a little kindness.
Offred is not particularly brave, neither is she a coward. She's smart enough, but not exactly ahead of the game. She can be caring but she's also looking out for herself. She's so middle ground that it's incredibly easy to identify with her, but also to distance ourselves somewhat.
I agree, that the fact she started her relationship with Luke whilst he was married, that's not written by accident, neither it is an oversight that we are not told anything more about the circumstances.
Most women would naturally judge a woman who goes after a married man. Even liberal Moira was judging her friend for not staying clear. I think she said something along the lines, you can't help who you fall in love with, but can help your actions. But of course we were not told any details about this affair. Perhaps Luke was desperately unhappy. Perhaps his wife had cheated on him first. Perhaps it was Luke who pursued Offred...
Spoiler:
Anyway, it's a clever twist to write her as someone who was at the least instrumental in the break up of Luke's first marriage. It just makes the reader realise how easy it is to judge. Knowing practically nothing about these people, and the judgement is already sizzling away.
This is a huge problem with relentlessly inforced black and white law, how to deal with theft, assault, murder, adultery, abortion... there are always circumstances to consider. Life is never black and white.
I think Offred's situation is doubly interesting, because of this untold backstory, and her status which is due to her marriage declared as non void by the new state of Gilead. In other words, she "deserves" her new status as a Handsmaid. Just like Janine was made to accept responsibility for being raped.