Don't worry about that you are being made out to be the bad person. So what. You know the truth. Now fix it. If you don't, it will get worse.
And tell the other woman to stop spending time with your husband. And tell her husband about all the texts you found. My question to him would be, why aren't you with your wife in the evening and at night?
As for this part, "Your husband, the other woman and her husband, CHRISTIANS? Uhm, I wouldn't say they are Christians. Christ wouldn't treat His followers the way they are treating you." I strongly disagree--they very well may be Christians. Christians often fall as fast or faster than non-christians, because the devil is out to wreck Christian marriages! We humans are MUCH weaker than we think we are, and thats why there should be rules.
Yes, show the woman's husband what is going on! Perhaps you will find out he really is using her phone to chat with your husband. Some men are chatty like that, not too many, but some. And demand that they stop deleting the messages. you have a right to be able to participate in the conversations by reading them after the fact....and to know that they have nothing to hide.
Do whatever you have to do to make this end...ignore how you may look to all of them--your marriage is at stake. but make your word stick! Hugs. I feel for you and will pray!