A person could not even walk through our front room any longer, let alone get to the bathroom!! I'm at the end of my rope and feel defeated!! Can you please help?
Your daughter's behavior must result in her receiving the natural and logical consequences it should bring. Her continued misbehavior and the "war" she has drawn you into are not about things being carelessly left around the house. Now this is about power, anger, control, revenge, and attention, among other things.
At a time when both she and you are getting along (never in the middle or aftermath of heated discussion about this matter), take her out for breakfast/lunch and calmly introduce this problem into your time together. Mention that you know you're both upset over this problem. You're upset with her and clearly she's upset with you. Tell her that you feel your only options at this time are to pick up her things that are a constant safety and clutter problem (leave her room out of the talk), box them up and then ask her which children's charity she wants to give them to. Indicate you will not be buying any replacements of these items.
Then ask her if she has any better solutions to the problem. Ask her what she considers to be a fair handling of this situation complete with her suggestions for logical consequences for her noncompliance. If she comes up with anything remotely sensible (suggesting you just leave her alone about this is not an option, there needs to be an action plan) say you'd be glad to give it a try for one week. If her plan is working (it doesn't need to be perfect but it does need to be a major change) praise the progress and her plan, saying how difficult it is for anyone to stop bad habits. If it's a total failure and she just can't/won't stop her "war" then you have to follow through on boxing up her things and letting her choose which children's charity they go to. If she won't choose, then you make the selection.
She soon will have few possessions to leave anywhere if she insists on maintaining her contrariness. You have to break the rhythm of this "dance". She hasn't been able to up to this point; her heels are dug in too deeply. So it's up to you. With this approach, at least the present rhythm will be broken and you will take this to another level. You might also be looking for behaviors of hers which you can comment positively on. Don't make a big deal of noticing, just be casually appreciative and encouraging.
Let me know how things progress.