In other words, where is the line between permissible and impermissible going back on your word drawn?
You are looking at it the wrong way. You are assuming that it is permissible to go back on your word and are searching for conditions which grant that permission. I approach it from the opposite end of the spectrum - it is not permissible to go back on your word, full stop, so don't give your word unless you mean it.
If Rubio can't keep his word on this, a simple pledge to support the nominee (which says in no way how much he is to support the nominee, simply not attacking is fine), then why should I trust him to uphold the oath of office? Because it is said on a bible? Not been many lightning strikes lately for breaking that, has there?