It was just that you were applying a law about vagi (someone without a home) to a situation about a traveller (someone away from home). It’s an easy distinction to miss if you’re not accustomed to looking for the distinction.
Vagi are bound by the laws of the place where they are actually present at the moment–where they “woke up” that morning isn’t relevant.
So - the upshot of this would be, if there is no Mass for him to go to on Ascension Thursday (keeping with our example even though that was almost a week ago, now) because they don’t celebrate Ascension Thursday there, he doesn’t have to attend Mass that day,
More-or-less. Since the universal law is exempt in the place where he’s visiting, he doesn’t have any obligation to attend Mass on Thursday–even if he does know of a place offering Mass. Note that I’m saying there’s no “obligation”, not saying that it wouldn’t be a good thing to do.
Your conclusion is right–he would not have an obligation to attend Mass.
but at the same time, if he goes there on a day of fasting that he is not aware of (like if they observe the Ember Days in that Diocese, but in his home Diocese they don’t do that, and it happens to be an Ember Day) he is not required to observe that, either.
Is that a proper interpretation, or have I missed something, still?
Again, more-or-less. As a traveller, he would be exempt from fasting in the place where he visits even though fasting is required in that place. Canon 13.2.2 This is true whether-or-not he’s aware of the fasting requirement where he visits.
He would ALSO be exempt from fasting even if his home diocese had that as a day of fasting because once he leaves his home diocese, he is no longer bound by the local law of his home diocese (that’s canon 13.2.1) because that particular/local law adds an obligation.
Again, your conclusion is right–he would be exempt from the requirement of fasting.
In anticipation of a question, even if both his home diocese and the diocese where he’s visiting are observing ember days and fasting in both diocese is required, he would still be exempt from fasting by virtue of both parts of the canon. 13.2.1 & 13.2.2
[unless not-fasting would be a cause for scandal]