You are very unlikely to get 100% hatch rates with ANY incubator -- the styrofoam ones can work quite well, but I do think they aren't the best incubators out there. I've got one styrofoam one, and the Nurture Right 360, and have been using both this year. The NR360 has a candling light on top of it -- candle your eggs about a week after you've set them, and see if they are starting to grow. You should see a network of veins in there. If it's clear, toss the egg at that point.
I've been doing 'dry hatch' this year -- it is actually dry incubation, because you do add water the last three days before they start hatching. It's working well for my chicken eggs.
And I almost never 'help' chicks out of their shells. If they can't get out on their own, they aren't strong and I don't want them in my flock, where I may breed from them. Usually, if the humidity is up where it should be at hatching (65%-75%), you don't get stuck chicks, so that shouldn't be the reason they are having trouble getting out. If you consistently have problems, you may want to look at what you are feeding your parent stock -- they may need a higher level of nutrition.
Fresher is better, but people have hatched chicks from eggs as old as three weeks, so don't give up on them until after they've been candled.
I was keeping all of my eggs in the styrofoam incubator until three days before hatching, then moving them to the NR360 for hatching. I had put both incubators away, but a friend gave me back a dozen eggs from a flock descended from some of my original Icelandics. I added a couple of green Lavender Ameraucana eggs from another friend, and a dozen bantam eggs, and filled up the NR360 (over-filled it, actually, but I'm sure I'll be tossing a few after I candle them on Saturday). This is the first batch that will stay in the NR360 all the way through, but I have been impressed with how well it holds temperature, and we like all the clear plastic, which allows us to watch the chicks hatch.
Kathleen