I don't know why it took me so long to realize I should be using my feeler gauges to level the platform. 
But here's the latest. Now that it's mostly flat, I'll worry about raising it to 0-ish tonight. Now time to start increasing resolution.
Edit: ugh, just re-homing the Z should get it to zero-ish. I'm clearly firing on all cylinders today.
My Z Probe section:
; Z-Probe
M574 Z1 S2 ; Set endstops controlled by probe
M558 P9 H1.5 F120 T6000 ;A5 B1 ; Set Z probe type to bltouch and the dive height + speeds
G31 P500 X-25.3 Y0 Z0.675 ; Set Z probe trigger value, offset and trigger height
M557 X25:200 Y25:200 S20 ; Define hi-res mesh grid <-- OMG I didn't know about the "P" argument... So many maths could have been saved 
;M557 X35:365 Y35:365 S330 ; Define low-res mesh grid
For the first time in 2 weeks, I made it to layer 5 on a print with no errors.
That elation tho.
Thank you guys for the help. All of the combined knowledge you guys shared should be part of a book, or wiki, or something for Duet, cause that was extremely helpful and useful. Seriously, @Phaedrux @grizewald @Veti thank you guys!
The glass combined with the solve on the X-gantry made a massive difference!