I have two 1602N hovabators and I love them. I use one with turners and a fan for the main incubator (days 1-18). I have another 1602N that i use for a hacther. They are really easy to use and control throughout incubation.
You can also try to candle the eggs, sometimes you can tell if they have internally pipped. Also sometimes you can hear them tapping on the inside of egg or see the egg moving around.
You really need a second incubator if you are going to try a staggered hatch. You can use the second one for a hatcher and the first one just for incubation. If you do it that way you can add eggs when ever you want to. I marked the dates on the large end of the shell that way you know how far...
I have never heard that increased humidity softens the shell. I had done a staggered hatch once and lost count of which eggs to lockdown. The humidity had slipped down to around 25% in the main incubator and I had a old english bantam chick actually in those conditions. The little one did great...