The first "dead end" does lead to an area, but it's for the pacifist route. When you qualify for it, phoenixes appear in the path to it, both drawing attention to it and refilling your stamina so that you can get there easily. I'm not sure it would be practical to make that crossing impossible otherwise considering how efficient spider form can be, but they probably should have put a gate the old woman could open so that it would be obvious why you can't continue, and definitely should not have put the save point somewhere a murderer can reach.
I have no clue what the deal with the second dead end is. I have been there in a variety of circumstances and never found anything there.
I agree that the softlocks are a major issue, and they make it impossible for me to recommend the game to other people despite how much fun I've had when the game works.