FWIW, your original version wasn't far away. You would either do this:
Less:
.m-faContent( {{$xf.options.sF_SubForumManager_unreadSubforumIcon ? $xf.options.sF_SubForumManager_unreadSubforumIcon : @fa-var-comments}} );
Or a shorter version is this:
Less:
.m-faContent( {{$xf.options.sF_SubForumManager_unreadSubforumIcon ?: @fa-var-comments}} );
Our preferred approach is to use the Less specific functions though.