I think you are on the the right track. I had to stop putting water in my bators around the first of june because i had a lot of pipping and dying and a lot of slimmy chicks as well well as dying before they pip. Now they are back to hatching good.
I don't really think humidity is the problem. I think the cool temps just slowed growth so much they just never got started. The biggest reason for humidity is in the last few days and after the first pips. It is too keep the membranes moist and tender to allow the chick to turn and pip out...