Julie, it is a really trying situation that you have described.  My
brother was living with someone like that and things were almost as bad
(although he left after a considerably shorter amount of time due to
other problems with the relationship).  Anyway, the best thing to do
would be to get everyone in the same room together (optimally in a room
with nothing breakable), lock the door behind you, throw the key out
underneath the door (just as far as the longest hand can reach.  You
would like to get out after the conclusion, I would imagine), and hash
things out.  More than likely, there will be screaming, crying, and
possibly hitting (unless of course someone decided to bring some rope to
tie people down).  Some of the best strategies in keeping things calmer
would include:
   have each individual own their own statements (ie, I feel that this
relationship is hurting everyone involved because.... or I really don't
understand where you're coming from.)
   reinforce statements by paraphrasing, etc. (ie, So you think that we
did this because of...?  Well, let me just say that the reason for this
was ....)
   don't accuse each other (It was your fault that ... happened!)
   find a common ground about SOMETHING (Lampshades really are
decorational and functional at the same time.)
   Guaranteed, in a situation like this, there is going to be some
gunnysacking (re-hashing topics which were assumed resolved, but were
truly not and someone feels someone else is to blame).  However, this
should be kept to a minimum and simply ask for forgiveness or apologize
about each situation WITHOUT holding a smoldering grudge.  

The relationship really can work.  It's just a matter of keeping things
smooth and even.  It's sort of like making a peace treaty between
warring factions:  you can't give one side everything; there must be a
compromise.  Breaks can be taken, but communication between everyone
involved must continue if the relationships here are to survive.