The two strongest female characters, Offred's mom and Moira, both have unhappy endings. If these two independent women cannot stand strong against oppression, what hope does Atwood leave for anyone else?

I am not sure I can agree with the assessment that Offred's mother and Moira are the two "strongest" female characters; they are certainly the most obvious and outspoken feminist characters, but that does not necessarily mean they are stronger than, say, Offred herself, Ofglen, or even Serena Joy. However, I would also argue that Moira, does, in your words, stand strong against oppression. Moira tries to escape Gilead multiple times, so her opposition to the regime can be understood in this way. We do not really know if Offred's mother attempted anything similar because the information Offred has about her is so sparse. When met with force and compelled to choose between life and death, what could we expect them to do? Either choice, in such a world, feels justifiable: neither one seems weak to me. In showing the time before the regime took over, when smaller changes to society were being made in order to pave the way for Gilead to become established, perhaps Atwood is showing us that it takes more than strong belief to retain our freedoms: strong action is warranted when we see them disappearing.