I know this would result in the loss of several days postings, but as the forum update seems to have been a complete disaster as far as I can see.
It seems logical,take the new site offline. Backup the database and files.
Restore the old database and files from last week, and put it back online
The setup a staging serve, for the new forum, restore the new forum DB and files to the staging server, and let people use it purely to get it working.
In a month or two when the new site is working, an update could be performed on the main site.
As a professional developer I have done updates on major commercial sites, and the new site is always trialled extensively on a staging server before its ever made live.