No you cannot edit Standalone. From BM's previous post

“Standalone” is one Quick Setup that you cannot customize. It simply returns the instrument’s Utility Mode settings to the most normal playable status. The other Quick Setup items can be taken literally they quickly allow you to make all the settings to do specifics tasks with one button push...

You can customize Quick Setups 1, 2 and 3... to suit your needs. But please understand these as just a shortcut so you don’t have to make the dozens of settings every time you want Record. The factory suggestions provide one for recording normal MIDI, one for recording MIDI output from the Arpeggiators, and one setup template configures the instrument for Out via the individual Audio Outputs via USB.

If "Standalone" doesn't do anything you want - you cannot change it - so do not press "Standalone". You can customize Quick Setup 1 or 2 or 3 (or all) to do something else. You may want to create your own "Standalone"-ish Quick Setup that retains your Single-channel setting.

OK, that's what I thought.

So I save my current settings to the first quick setup slot and rename it. Can I select that quick setup as the automatic settings for when I power up the MODX or I am going to have to navigate in and select it every time?

In the latter that case, I don't see the advantage over just having to select single mode upon startup.

So I save my current settings to the first quick setup slot and rename it. Can I select that quick setup as the automatic settings for when I power up the MODX or I am going to have to navigate in and select it every time?

No. But if you were using the Zone Master setup to interface with your external device, you could teach your MODX to boot up on your favorite Performance (the one that contains your external device setup). See how that works!