Which of the two fatwas should the questioner abide by: the fatwa of the mufti who ruled that what happened was an oath requiring expiation, or the fatwa of the scholar who ruled that divorce occurred?

You are not obligated to follow a specific scholar, but you should adopt the more cautious opinion and follow the more pious one among them. The view that the oath of divorce (yamin al-talaq), when divorce was not intended, has the same expiation as an ordinary oath (kaffarat yamin) is a well-known opinion among some scholars and is relied upon in most courts. If you wish to adopt the more cautious opinion (that of the majority of scholars), then if your wife bought something from her salary, the divorce has occurred. You can take her back as long as she is within her waiting period (iddah), provided it was the first or second divorce. You should avoid such oaths and resolve disputes through dialogue.