I'm not sure what that output format > channels > one channel should actually do. I don't think it's intended for what you are trying to do.
If you have a channel imbalance, then presumably you either want to select one channel, or you want to add the two channels together. I don't know what the above does. To do channel mixing, you would use a a Parametric EQ action for working with channels.
I think MC is probably the wrong tool for this unless you have tons of them to do. Audacity might be a better choice. It does it's work losslessly and you can see the inputs and outputs as a graphical representation so you can verify what you are doing. Audacity can save back to MP3 if you'd like. If you have a "channel imbalance" where (for example) the left is louder than the right, Audiacity makes it pretty easy to copy one channel to another so they become identical.
I question why you want to 'save space' though. There's no reason to these days. MP3s are already tiny. Even FLACs are so small that there's no reason to try to make them smaller. Hard drives are extremely inexpensive.
I still don't quite get what "two channel mono" is supposed to be. I guess you probably mean a stereo track which has identical signals in the left and right tracks. This to me is simply stereo that was sourced from something mono. Two channel mono isn't a term I've ever heard and I've been doing audio in one form or another for about 40 years. Jeez that makes me sound old. lol.
Take care,
Brian.
