1. When placing a thread on a trad route or in a belay, is it acceptable to use a double overhand? Or is a double fishermans the only acceptable thing to use?2. I've got some skinny cord that I sometimes use in trad belays and I often wonder whether a double overhand is advisable with this skinny cord. Might be more prone to rolling.
3. If placing a thread for use on a trad route, what's the minimum diameter cord/rope that you'd use?
Personally if I were placing on ab and not on the lead I'd go for double fisherman's in nylon, triple in kevlar/aramid/dyneema
Real world answer: the strength of the placement and any sharp edges therein may often make the above irrelevant.