BuddyPress.org

— Due to the privacy issues, and to be a member on my site, you need to pay a fee, —

99.9% of those who help out in this forum are volunteers. You cannot expect volunteers who are trying to help you to pay a membership fee to troubleshoot your custom theme which is already going beyond getting BuddyPress working in your site.

— is it absolutely necessary to be on the front end? —

It would make it much easier and faster to identify the problem.