is there a problem with using a very thin bowstring with conventional nocks ?

if the arrow stays in place until release, would there be safety or accuracy issues after release?

I don't think there's anything to worry about.  Essentially all my archery experience is on the primitive side.  All of my arrows are wooden and handmade, so the nocks are just cut out with a saw.  Sometimes there's one that's a little wider than others, or just slightly off center.  None of them "clip on" to the string like a modern nock can.  But I haven't had any trouble with them.

Just as you say, as long as the arrow is held in place until release, shouldn't be any problem.  Does the string have a serving?