The
Defiant's crew probably died of their own insane actions long before any physical pathology. The physical distortions might have led to death eventually. What McCoy did was come up with a "pain killer" to numb the fun house mirror sensations—temporarily.
Gravity boots? No mention of that in TOS. Anyway, since the
Enterprise sensors could not detect the
Defiant, it might have been some kind of mirage. I think the environment suits were a wise precaution. They might have been beaming into space. The ship appeared to have power, although the air might have leeched away. McCoy passed his hand through a table and one of the corpses. I always figured that the air was intact, and McCoy had wandered down to a threshold between the universes.
To coin a phrase, "'brane and 'brane! What is 'brane?"