"If 'mind' acts on body, then all physical laws are invalid." [...]

The last quote suggests an explanation, if not a justification, for his position. He could not solve the question "what is mind?", and hit the Ignore button.

Again, this seems to be simply an expression of materialism. What Watson was saying was that there's no mystical mental essence that intervenes in violation of physical laws to control the body. Just so.

When Watson says "mind" we should usually translate that as "spirit" or "soul" - if we actually want to understand what he meant. 'Mind', 'consciousness' and 'soul' were all words from the dualistic psychology he was trying to administer a death blow to. The point he was making was that that stuff isn't real, it's all actually down to physiology.