Aw fluffydown...so sorry you're going through this. Sometimes it happens this way.
Do you have any kitchen sponges that are clean? If so, cut them into slim pieces. Warm them up in HOT water. Put them soaking wet into the corners of the incubator. Only fill up the incubator with warm water, otherwise you can cause the temperature in the incubator to drop and take forever to reheat. If you've been using lukewarm water, this may be part of the problem.
If you have a mister or a spray bottle, put some hot water into the bottle and spritz the eggs. This will raise the humidity ASAP. The day 18 stuff is just jargon. It's best not to touch it, but you won't murder chicks that are hatching. Just spritz the eggs with the mister/spray bottle and they'll be fine if you open it. I've done it forever and haven't had an issue.
Another thing is, those styrofoam incubators are gambles at best. Don't beat yourself up about it! This stuff happens and the previous owner may have known about temperature fluxes and not disclosed them to you.
Please don't let this be the last time you incubate. It can be very rewarding.
