The OP makes excellent points. Mu's prospects seem to me to be slightly stronger in the 400 right now than in the 800, particularly if Shaunae Miller-Uibo isn't running the 400. I agree that being able to run in one's own lane greatly reduces the likelihood of a "left-tail" event such as falling. As the OP put it "if your equally good 400/800 run the 400" (sic).
Moreover, if Mu runs the 400, the schedule will allow her to also compete in both the mixed 4x400 and women's 4x400:
Friday July 30:
Mixed 4 x 400m Relay Round 1
Saturday July 31:
Mixed 4 x 400m Relay Final
Tuesday August 3:
Women's 400m Round 1
Wednesday August 4:
Women's 400m Semifinals
Thursday August 5:
Women's 4 x 400m Relay Round 1
Friday August 6:
Women's 400m Final
Saturday August 7:
Women's 4 x 400m Relay Final
This is a demanding schedule but is well within the wheelhouse of a 400/800 runner like Mu.
By contrast, if Mu runs the 800, she will be unable to compete in the mixed 4x400 finals. The mixed 4x400 finals are immediately after the women's 800 semifinals on Saturday July 31.