I think you are Ok now.
If you study the linkage....and slowly move it thru it's travel (engine off), you will see that the carbs begin to open, yet on another part of the linkage (under the flywheel, right side) the timer base begins to move too.
All that has to be synchronized to happen at just the right time.
Generally, best cold starting occurs when the timing is advanced, carbs are open very slightly, and you have primed (choked) the heck out of it by pressing the key in. Each engine is different, tho. Warm starting, none of that should be needed.
You don't need to assume that the butterflies are closed at idle and that they operate in unison....just look for yourself.
You can inspect them very easily....get a flashlight, and operate the butterflies at the top where the cam roller touches the cam. You will see it if you study that linkage.
But, unless you know what you are doing, don't make any adjustments.
Don't worry about those little hair cracks...I see them just like you describe all the time. If they open up, different story.