No, corner clipping does not prevent fraying over a long length of cloth, as I can attest with wads of strings in my washer and dryer.
I serge all my edges.
As to the problem of the serger not working, try to check out the lines of threading through the machine. 9 times out of 10 for me that is all the problem.
And when I change thread, I just tie the new thread on before the thread enters the guides without changing any settings, and then just sew very slowly and the little knot follows right along until it comes out behind the needles, new color thread now in place.
Never had a problem with this, and rarely ever have to actually rethread.
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