You say that you can't block OW's number on your wife's phone. This may or may not be true; I have AT&T and can--for a fee--block up to 30 (maybe 40, I don't remember) numbers.
You also say that, because of the number of contacts, it would be too hard for your wife to change her number.
This is where you lose me. Our phones now hold so much information that a phone number change is simpler than it has ever been. People do it all the time, for a million reasons. All you have to do is send a mass mailing to her contacts (on her phone and in her email---all of which are likely right there on that single iPhone) and say, "From this point forward, my new number is xxx-xxx-xxxx."
No need for explanation; people often change their phone numbers. It brings peace.
Then you hand the iPhone to the guy at the AT&T store and say, "Disable this number and enable the new one." Go with an unlisted number for the new one. No, it's not fool-proof, but it makes things trickier. (I changed my number to avoid sociopath family and ten years later, neither has managed to locate my home or my phone number. It's quite possible to lose psychos, even if some of the actions required are unpalatable.)
Look, the consequences suck---they really do. As a BS, it really does feel terribly unfair that we have to shoulder ANY of these consequences. But if we want peace in our lives, we simply must do what we can to mitigate the damage---even if we're not responsible for any of it.
You invited this woman into your lives---but you can help evict her. You really can. You can reassure your wife that it will be well worth the miniscule inconvenience of changing her phone number--that every contact she has will simply copy and paste her new info into their smartphones---just as she would do if a friend or acquaintance changed his/her number.
If reasonable, relatively simple actions like changing phone numbers do not work, then an attorney can send a cease and desist letter.
Forward each text message your wife has received to email and print it twice---keep a set of copies in separate places so that if something happens to one set, you have the other. Thoroughly document the harassment so that you can get appropriate restraining orders.
But please. No more
nice NC letters, gruff NC letters
. YOU must maintain NC 100 percent. Each "NC" letter you send gives the OW exactly what she wants
Help your wife find ways to keep OW out of her life. Make it as easy for her as possible.
This is the small stuff...but helping her with it really, truly helps build trust that you can help with the big stuff.
Good luck. It gets better.