After reading what you've written, with all that math, I'm feeling tired 
This is trickier than I thought 
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Would using vnstat (on router) be an easier way to approximate my usage? I understand that vnstat monitors "LAN communication", but since I don't do any LAN communication (e.g. no peer-to-peer file sharing, LAN gaming), I don't mind paying the 100% of what vnstat calculates.
Please see comparisons of my vnstat and my nlbwmon here: Why such a huge discrepancy between vnstat and nlbwmon's bandwidth stats
The communication between my devices with the internet via wifi.