Question about the superposition-theorem
Superposition says that the solution to a linear circuit with multiple sources \$A\$, \$B\$, and \$C\$ is
$$
S_{ABC} = S_A + S_B + S_C
$$
where each \$S_X\$ is the solution with only source \$X\$ active.

You could also write this as
$$
S_{ABC} = (S_A + S_B) + S_C = S_{AB} + S_C
$$
where you've combined sources \$A\$ and \$B\$ before considering \$C\$. That's fine.

What you did was you skipped straight to the last expression: you found \$S_{AB}\$ (without superposition) and used that to help you get the solution. Looks good to me.