Joseph,
Regarding the expiration of the Cherem, see the below link, the pishchei Tshuva quotes many rishonim and poskim that do not even mention anything about having a expiration date. Basically, it’s a machlokes and we go according to the other shitah.
http://www.hebrewbooks.org/pdfpager.aspx?req=14398&st=&pgnum=12