"I can't use my external display during the process?"...
As long as you have video externally through the hole process, you will be okay

When I need to use my external display, I right click my desktop, and go to 'nVidia Control Panel" where I can select whether to use the display or my laptop screen. After I select the one I want, there's a 15 second preview period. If I want to keep using the display I selected, I must click yes or after the 15 seconds period, it goes back to the previous screen used.

I went to the control panel, and chose 'laptop screen' as my default selection, but I'm unable to click "Yes" on my laptop because I can't see anything (due to the black screen). So it reverts back to my external display.

If I could get around this problem, I might be able to fix the problem entirely.

use the left or right arrows on keyboard and hit enter

Your gonna have to carry your stuff, tough luck my friend.. good luck

Thanks for the continued help guys.

I used the arrow keys, as suggested by tkr3alm3r, and it the display switched to the laptop BUT the screen remains black/off.

I know it switched to the laptop because my display screen went into standby a couple seconds later, meaning it was no longer in use by the laptop.

Are there any commands I could run or anything I could do to fix this problem?

Thanks again.