Re. Beckfish sync client
I have been chatting with the developer - I think s/he did not see some of the differences between syncing with Sharepoint Online and with Sharepoint 2013 (the latter does not allow characters { } ~ and & in the file path and has a shorter overall maximum address length). It was coded with the stricter set of validations only.
See support.microsoft.com/.../2933738 for some of the differences.
I have suggested some ways to get around this, though no ETA on when those might be worked on. Given s/he responded to my original query inside of an hour, I think it could be pretty quick.