I think I am understanding punctuation with respect to intonation in a direct address.
Intonation: the rise and fall in the sound of your voice when you speak.
No comma after "Go" indicates that my voice did not increase or decrease in intonation, but rather it remained at the same tone when I uttered both words; hence no comma after "Go".
The comma indicates that my intonation dropped slightly when I said Yankees; thus the comma after "Go" would indicate this dip in intonation when pronouncing Yankees.
Am I correct on both counts, i.e., my punctuation in each example and the reason/logic for each?
This is the place to post questions and discussions on usage and style. The members of the Wordwizard Clubhouse will also often be able to help you to formulate that difficult letter.
End of topic.