Since I have a number of 135 spaced wheels on hand, the first thing I would do
is swap wheels. If it stops skipping, then it was the freewheel that caused
it. If it still skips you would confirm the freewheel is not at fault. To your
point: things skip when they don't fit together correctly.
Have you tried a different derailer? That Sora looks pretty stretched out.
I'm using a silver version of the below linked M-591 SGS Deore and I've
slammed a 135mm Deore XT hub in my dropouts without respacing, using
Shimano HG61 11-34 9 speed cassette, Microshift bar-ends and whatever 9
speed
