Solved my issue. Apparently mattercontrol won't do anything the same size or smaller than the nozzle diameter. Model was 0.4mm walls, changed the nozzle diameter to 0.39 and it slices fine. So I made my own quick model with 0.5mm walls which worked fine from the perspective of getting it to print.

However I'm at a bit of a loss again as the gcode says my extrusion width is 0.4mm but the resulting object is measuring 0.8-0.85mm. I'd just re-done the e-steps test and verified my values before this print so I can't see it being out that far.

Put a multiplier of 0.48 in and end result was a 0.45-0.5mm wall thickness.

So confused.