Controlled impedance and differential pairs over M.2 edge connector
If the difference in impedance is sufficiently small (15% isn't bad for digital) and sufficiently short (USB 2.0 needs ~240MHz of bandwidth while ethernet needs a bit less). 

Generally in the RF world you can get away with discontinuities less than 1/10th of your wavelength of interest (in this case 240MHz, make it 1GHz to include the third harmonic and some extra margin).

A 1 GHz signal has a wavelength of ~30cm in air or around 20cm on a pcb (higher dielectric constants shorten things). So that's still 2cm if we go by the 1/10th rule of thumb. I'd imagine the m.2 interface would add less than 1cm so I think you're pretty right here either way.

We can even calculate the reflection co-efficient if you want to be doubly sure. It's just (Zl-Z0)/(Zl+Z0), which'd only be ~0.08 when transitioning from a 100ohm trace to an 85ohm one, consequently this corresponds to a VWSR of ~1.2 (which is more or less on par with more than a few cheap RF connectors) and means that your return loss is ~20dB (so about 1/100th of the signal power gets reflected). For digital signals (especially sub-Gbit ones), that sounds alright to me.