I think that "surface offset" equals "shell".

One technique is to make the object a little bit longer, and trim off excess. But first the edge fold (surface fold), must be prevented.

Zooming in, it seems that the shell is causing an edge to fold back on itself, so avoiding shell would seem to be in order.

-Brian

ps, I Like your design.

I did move the trim plane by a tenth of a mm, or so.