So many pieces to Leatherhead's story... Many different odds and ends to sort through... I've always went with the notion his actions in "Leatherhead Too" were caused by the illuminated's drugging... Causing him to subconsciously change plans and do things he normaly wouldn't... Perhaps it was early on, so his eye hadn't healed yet and the growth aspects weren't in effect? Maybe the illuminated were responsible for him finding those supplies too? As for his rage in the flashback story... Perhaps he was just young and still learning the dangers of his abilities? The panel where he bites Splinter is more of a foreshadowing to his mutation being actually caused by exposure during their fight... At least that's what I get out of it...
Leatherhead's had many different writers over the years... Perhaps it's only natural things don't always fit together perfectly?