Relative to impedance, just clean the electrodes and clean you forehead each night using alcohol wipes, and you'll get a good signal for quite awhile. I'm personally not sure what is a good typical impedance number versus a worsening one. The usual sign that the headband sensors are going bad is that you consistently get alot of zeros in your Base Hypnogram (except at the start of the hypnogram where that is normal), or that the Zeo Alarm fails to even acknowledge the headband has detected any sleep.
Given you have issues falling asleep and lack of deep sleep (the exact same issues I have), you are going to find some other things out about the accuracy of the Zeo. The folks at Zeo only claimed accuracy for those persons WITHOUT any sleep issues, and they didn't publish results of their accuracy for persons with various types of sleep issues. In particular, the Zeo (and almost every other sleep monitor including formal sleep clinics) have significant trouble detecting the moment when one falls asleep. The EEG does not have some type of sharp change the moment one truly falls asleep.
When you read the details of the Zeo that they published, you'll find they approximated the point when the Zeo says you fell asleep (the Time to Z parameter). Basically its set to around 5 to 7 minutes after you settle down and stop moving significantly (and the Zeo Alarm headband version does not have an accelerometer). So you are going to find that you are laying in bed quite awake, occasionally looking at the clock, yet the Zeo is going to state you are in either REM or Light Sleep. I've found the Zeo is not very accurate for persons that have issues with falling asleep, as opposed to persons that have other sleep issues (like overnight wakeups, apnea, RLS, or short sleep).
However in counter-balance, you will also begin to find that the duration of time that you think you spent awake trying to fall asleep you were actually phasing in and out of very light sleep. Your train of thoughts will continue as you phase out of the Light sleep just as if you truly were awake the entire time, but you are experiencing something similar to a "drivers trance" and you've spent more time asleep than you thought. But because of that excess Light sleep, you've got a corresponding lack of Deep sleep (which is my symptoms as well).
I don't have an answer for you about dealing with that, since I've not solved it for myself. And you are very young; but my experience is that these symptoms will get worse as you age. So working on these issues now for you sleep is a good head start.
If you dig around the web, there are many people experimenting on sleep supplements, sleep inducing technologies, changed sleep environment, sleep meditation techniques, etc. You are just going to need to try different ways, and get your significant other to help you with your sleep. However, there is always just simple adaption time ... your body/mind should grow more "comfortable" with your others presence over time, so these sleep issues may fade over time naturally.