See the person next to you? Kill! Kill!!! KILLL!!!!
I'm your baby! Can you feel it?
Yes, I believe you have it right - it should be "must".
I don't have the rules in front of me so I'm not sure:
if Argos is empty: then no, don't have to continue. The first movement is an attack, you must roll the dice until you have progressed at least once but you don't have to continue after that.
If Argos has troops of your own: then yes, when you add more units you must continue the campaign for at least one progress.