Of course, the ambiguity allows you to believe that one, both, or neither is a Thing.
Personally, I believe Mac's "let's see what happens" means he's going to watch Childs, and hen blast him with a hidden flamethrower or something after the credits roll. Just my take.
Nope. Neither were. Remember the blood test? Every part of the thing was a living entity that struggled to survive. What does alcohol do to tissue? Why did Mac seem to relax after Childs took a drink? Because if he was a thing, his mouth tissue, et al., would have reacted against the alcohol. It was all a test by Mac to see if Childs was a thing...