What is the ruling on acting upon a weaker opinion, and is acting upon it considered invalid, and is the one who acts upon it sinful if the stronger opinion is prohibition and the weaker opinion is merely disfavor (karahah)?

The basic principle is the obligation to act upon the preponderant opinion for someone who knows that the other opinion is less preponderant. However, if acting upon the less preponderant opinion removes harm or hardship, or achieves a religiously valid benefit, then scholars have mentioned the permissibility of acting upon it, and the necessity is estimated according to its extent. Similarly, it is permissible for a follower (muqallid) to act upon the less preponderant opinion for himself if no preponderant opinion is apparent, and if acting upon it does not combine permissibility and prohibition.