Is it obligatory to abstain from eating meat and dairy, and is it recommended to do so, if the slaughter is not Islamic or the animals are mistreated, and would that be considered a prohibition of what Allah has made lawful?

Islamic law commands excellence in slaughter and providing comfort to the animal. The butcher's misconduct does not prohibit eating the slaughtered animal if the ritual slaughter (dhakāt shar‘īyah) was properly performed, involving the flowing of blood and the cutting of the trachea, esophagus, and jugular veins. If the questioner knows that the animal was not slaughtered according to Islamic law, then it is not permissible to eat it, without generalizing the ruling to prohibit the meat of an entire country, especially in Muslim countries. What is doubtful is permissible, following the general practice of the people of that country. As for milk, drinking it is undoubtedly permissible.