Thank you for the detailed critique! I updated the game page earlier with some details about our experience making Divine Intervention, and your comments are absolutely spot on, I think we would have had a much better game if we abandoned the verticality and went with a more Contra-like side scrolling level, and some tougher enemies to go with it.
There is an ending trigger after defeating a pseudo-boss (just a bigger enemy with more HP :D) but you're right, the map was difficult to navigate and the end is hard to find.