Nehemiah praises God who keeps covenant "with those who love him and keep his commandments" (1:5), then says they have not kept them (1:7). What is he leaning on?

He doesn't claim that he or the people have kept the commandments; he says the opposite: "We have dealt very corruptly against you, and have not kept the commandments" (1:7). The prayer leans on three things, all in its own words. First, what God is: "the great and awesome God who keeps covenant and loving kindness" (1:5). Second, what God said to Moses, both the scattering and the gathering: "If you trespass, I will scatter you among the peoples" (1:8), "but if you return to me, and keep my commandments and do them … yet I will gather them from there" (1:9). Third, whose the people are: "Now these are your servants and your people, whom you have redeemed by your great power and by your strong hand" (1:10).