Admire your last post, catmumbaby.
Can I ask a question? I don't like the sound of this godmother/best friend and the other people who tell you all about this birthmother's life. It is really S* stirring, adding more tension between you and the birthmother. And that's nasty, because it will backfire on your stepson.
Who is to say that this same godmother/best friend is not going back to the birthmother and dishing the dirt about you and dh? Perhaps that is one reason why she comes across as being so 'interfering' as she is has been wound up and made to feel worried about how her son is being looked after at weekends. It sends alarm bells when you say she has asked for her best friend to be guardian, not her dh.
I think this godmother has far too much power in this set up and is abusing it badly.
Do you and the birthmother have much direct contact? Can you pick up the phone and speak to her yourself or is communication always through a third party - your dh, the godmother or others? I get the impression you actually don't know her very well but hear all these bad things about her. And if that's true, the same might apply to her.
If so, in your place I would find ways of ending this asap. It is not fair on you or her. Why should this godmother and others be in possession of all the knowledge - it is these people who could well be the controlling ones.
Can you, your dh and the birthmother do some things together with your stepson - just so he can see that all the adults in his life do get on with each other? It might ease the tension if you, the birth mother and your dh all have a better idea of how you each relate to your dss. Or is this a pipe dream? I have not been in this situation myself. As you met your dh after he broke up with his partner, hopefully there is no deep seated grudge against you - or is there? I know it must be very difficult.
If meeting up in the threesome is really not a good idea, can you meet up with the birthmother alone and somehow get to know each other a bit better? If you find some practical reason to do this ie plans for the school summer holidays, and then just take it from there, it might help?
And if you begin to get a better and closer relationship with the birthmother, watch carefully how the godmother and other gossiping adults take this turn of events. If they seem less than happy, they are not true friends.
I think caligula's posts on here have been really useful - great advice, btw.