Your sentence has problem, and I believe it is wrong:
- I could visit you yesterday, but I didn't want to.
Because you need a perfect here:
- I could have visited you yesterday, but I didn’t want to so I didn’t.
That way it is more clearly addressing an event in the past, just as this is:
- I was able to visit you yesterday, but I didn’t want to so I didn’t.
As for your second question, in this sentence, it is clearly a non-past situation:
- You could ask me before calling him.
It’s like saying:
- You would ask me for his number [. . . if you wanted to call him].
- I could do that for you [. . . if you asked me to].
"Rather than" can be followed by a gerund or a bare infinitive. (There are exceptions when it the two verbs it connects follow another verb that is the main verb of the sentence, but I'll ignore that case for this answer).
To take an example first of something that can't be used with such non-parallel forms:
*People might embrace their fetish or attempting treatment.
People might embrace their fetish or attempt treatment.
People must choose between embracing their fetish or attempting treatment.
Here or does require parallelism between verb tenses, so the first above is incorrect, while the other two are allowed.
Another informative incorrect case is to actually be parallel with a to-infinitive form:
*I would prefer to embrace my fetish rather than to attempt treatment.
I would prefer to embrace my fetish rather than attempt treatment.
Here we have to actually avoid parallel forms.
To come back to your question. In this case we can use both the gerund and the bare infinitive.
In all, either the form you found, or the form you suggest can both be used.
Edit:
It occurs to me that the distinction is hidden by the simple present (when not in third-person singular) and the bare infinitive look the same in English. It could be clearer in the past tense. First a case that does need parallelism:
He embraced his fetish or attempted treatment.
Here we posit two possible actions, that may or may not have taken place at some point in the past. We can even move the subject into what is parallel:
He embraced his fetish or he attempted treatment.
However, it would be wrong to have the same parallelism with rather than:
*He embraced his fetish rather than attempted treatment.
We are not saying that an attempt of treatment was something that happened in the past, or even that it is something that didn't happen in the past (though that is obviously implied), rather we are giving the possibility as one that hypothetically existed when he "embraced his fetish". It isn't something that exists in a moment of time, but adverbally modifies what is saying about the act of embrace, and no more has a tense than would an adverb like gladly, etc.
He embraced his fetish rather than attempt treatment.
He embraced his fetish rather than attempting treatment.
Best Answer
"Itself" is not used in this way in British or American English. I suspect that it may be used in other Englishes: Irish and Indian come to mind, but I'm not sure.
It does seem to fill a hole, as there is not an obvious word in UK or US which fulfils the same role, of emphasising the time.
I would say "just yesterday" and "just this morning": these are not quite the same meaning, as they are emphasising how recent the time was, whereas I interpret the original remarks as emphasising the time for some reason but not necessarily its recency.