You are correct in theory about the deep discharge, but what you fail to realize is that your device already contains a battery management system (BMS) that takes that aspect into account *to some extent*.
What I mean is that when your device will say it's fully drained, your cells won't be at a resting voltage of 2.5V. I don't have a specific figure but it might likely cut somewhere between 3V and 3.3V. So it might not be exactly the 80% discharge point but it will not be 100% that's for sure.
So do drain it more, have no fear. I also babysit my pricey cells that I use for RC stuff, but for 18650's that cost under $5 a piece I just don't care. Besides if you were to really follow all recommendations, you would be charging only to 80% and discharging only to 20%, giving you a safe range of 60%... nobody does that, it's just a PITA and not worth it.
For the battery gauge issue, it's a notoriously hard to tackle problem. Even ready-made IC's give very mixed results to say the least. I also dislike the way it's implemented in these vapes but you'll get used to it. Just read the value when the heater is disengaged (long 5 second button press when it's turned on)
Lastly, are you using an external brick charger or a USB port? I measured current draws upwards of 0.9A on my USB port when charging my SwiftPro (equivalent to CFV) Several users reported that their device seemed to never reach full charge on the meter unless they used a dedicated charger. USB2 ports are rated 0.5A max usually, these devices are pushing them too far.