Always_Ask_Questions,
“This learnt discussion is well above my head, but it convinced me to avoid Brooklyn on shabbos!”
Please do not feel that way, most of the people arguing here do no not know much more than you. Why would you avoid Brooklyn on Shabbos, you do not need to carry?
“My question – given the subtlety of the issue and, in some cases, lack of clear psak – is it all 1/0? That is, if we have some allowing, others – not, would carrying be always considered hillul shabbos by those who do not allow, or it maybe something like “ok b’dieved” or “ok when going for a mitzva” or “ok when not observed by public”?”
As the issue is possibly a matter of a d’Oraysa, there would be no heter. However, there are so many reasons to allow, and each reason would allow lchatchilah, hence, the argument to abstain from carrying is very weak.