You explained to your mother that you aren't emotionally ready yet but will be later, and this results? I hope you're coping, because I know I would be struggling!
The two texts are very disregarding of your boundaries (steppies? Really?) They have lots of self-justification and both tell you what to do, and at no point do these messages show that the senders care about you at all. The comment from the boyfriend about how your mother was unhappy until he came along and saved the day is just hurtful, especially when you are still coming to terms with the divorce. And the implications he makes are plain nasty.
They may not have intended things this way, of course, and may just be clueless. But in any case, they have blatantly disregarded your feelings in the situation and haven't considered you at all in all this.
You need to show your mother the texts. Find out if she gave them your phone number or if it was sneakily taken (she'll need to know this ASAP). If she gave it, then she may not have been clear on what you explained to her, or perhaps the boyfriend convinced her otherwise. It looks very possible that there is a lot of pressure coming from him in all this. Explain to her again that you aren't blaming anyone (unlike what the boyfriend's message rather rudely states) and that it's just a big change and you need time emotionally to get used to it. Pushing things to go too fast will destroy any chance of a good relationship with these strangers.
Please keep us updated if possible. This is an awful thing to have happened and I really hope it's resolved easily.