As far as I can read into what you are asking, I see a mono out of the Mackie into the soundcard.
The Mackie will have enough level ( perhaps too much level ) to feed both channels of the soundcard.
The Mackie is unbalanced output ??
Impedance matching and PAD may help with noise issues but generally I see no reason why this mono output won't drive two channels of the soundcard.
If the Mackie were balanced out you could record both halfs of the balance line.