C., momma, this too shall pass. Right now he's working really hard to support you all (VERY commendable). He's a stand up man who's taking responsibility for his little girl (I LOVE that...because sadly, not all men do) and recognizes how important it is for him to be a major part of her life. He's not selfish with his money (like a LOT of men would be in a similar situation like this).
On the flip side, its hard for you as well because this is a VERY isolating situation for you--with him being gone so much and your home dealing with day to day stuff and both of the kids (you are also very selfless for taking on the role of step mom, handling the little girl's day to day needs and caring for her as your own, especially when the mom is being SO difficult, to put it nicely). You guys really seem to have a good handle on things and just need to hang in there and wait for the wonderfulness that is waiting on the other side of it.
Try doing at least ONE THING a month alone, just the two of you. This way, its something you both get to really look forward to and it will help with the monotony of the day to day stuff with him working and you caring for the home and kids. Also, please be sure to keep track of the living arrangements for the child and make sure that the court is aware of it ASAP. In the end, not only will your man NOT have to keep ponying up to this woman, but he may end up getting a reimbursment for the money he's paying when its not necessary (because she lives with you guys).
On the topic of the overall situation, I've been there and know first hand that it can be SO lonely. He's most likely feeling pretty of lonely too and missing you guys just as much as you all miss him. You're on the right track so keep on keeping on. I have a pretty good suspecion that you guys will make it through it and be a stronger couple and family for it.