Hi Rachelina,
What I'm hearing is that something needs to change, but you are all hesitant to make these major changes.
I sincerely hope you use the links that Kevin provided to find yourselves a poly friendly therapist. This will be absolutely essential if you are all going to remain under one roof.
It's good to hear that there may be other living options in the future, including your in-law apartment and the floor inhabited by your husband's teenagers. I hope that in the meantime, the garage conversion helps matters.
Essentially, what it comes down to is this: if you (any/all of you) aren't willing to move further out of town, or have less space, or move the teenagers off the floor they are taking up, or move your tenant out (understandable and kind), you are committing to living together and working it out. This means self-work, forgiveness, acceptance and communication. This means teamwork. Ideally, this means going to therapy too.
However, we all have our limits. Just because something is thrust upon us, doesn't mean we are forced to stay in that situation. Conversely, it also doesn't mean that we have to resist that situation forever just because it was unplanned. The limits lie within ourselves.
My suggestion is that you turn your attention inwards, not outwards. Peace comes from within. Look at what you can do differently. You could start working on forgiveness. You could start opening more gentle and constructive lines of communication with her. Sit down and say "I really want to make this work. I appreciate XYZ about you. Would you like to talk with me about what went wrong, and whether you'd like to fix this?" You could start accepting that you really aren't ever going to be happy like this and accepting that you may need to leave for a while, or insist that she leaves and see how that goes down. You could decide that her behaviour, regardless of yours, is not something you want to deal with any more. You could express that and offer that either she moves out, or you will.
I hear you that acceptance is hard. It is. It's especially hard when it involves losing something (in your case, monogamy). It's too easy to remain in the denial/anger stage. You are actually grieving monogamy, so why not look up the stages of grief? Research how to move past grief, as if you were grieving a person.
With her, it's time to at least try to bury the hatchet and productively work together. Look up non-violent communication. Look up fair fighting. There's a website called Angries Out that has a great section on resentment, blame, and all the other crap we tend to throw at people. I'll see if I can find it for you and link it here.
In terms of your Buddhism... haha. Well, you are accepting being the world's worst Buddhist
Why not spend more time reading and following what you find there? Find quotes that inspire you and turn to them when you are feeling angry.
It is easy to blame others for our situation. It is also easy to feel that we don't have a choice. Change what you can change within yourself: your perspective, or your boundaries. If the others will work with you, you might all get somewhere together. If the others won't work with you, you will have to make your own way. But either way, it's time take control of making your own changes.
(edit) Ok, here are the links:
http://www.angriesout.com/rage.htm
http://www.angriesout.com/grown20.htm
http://www.angriesout.com/index.htm#couples