To answer your Bonus Question, what 7th chord?
‘Dm, G, C, F, Bb, E, Am, A’
The idea to creating a melody is to use the notes of the key of the chord. The same is true for choosing a minor or major 7th. This will change the notes used specifically in the Bb chord and the E Chord & A Chord because you are thinking in the CMinor mode key of bVII and the FiveOf satellite key AMajor & DMajor.
“The best I could come up with is ii, V, I, IV, bVII, V/vi, vi, V/ii in C major”
D,F,A,C = DmMin7 = ii = C Major Key
G,B,D,F = GMin7 = V = C Major Key
C,E,G,B = CMaj7 = I = C Major Key
F,A,C,E = FMaj7 = IV = C Major Key
Bb,D,F,Ab = BbMin7 = bVII = C Minor Key
E,G#,B,D = Emin7 = V/vi = A Major Key
A,C,E,G = AmMin7 = vi = C Major Key
A,C#,E,G# = AMaj7 = V/ii = D Major Key
Now these choices of 7ths is based on your interpretation. For instance if you were to think of the Bb as a IV/IV(the fourth of FmajorKey) the notes would become
Does this make sense?