Hi Claire,
I joined as I wanted to comment on your post. I've been married to a US serviceman for over 30 years, I'm sure my parents were very worried when I first met him that I would end up in the US, but they never ONCE made me aware of this as my happiness came first, not theirs. My Husband loves the UK and has retired from the military here now, although my parents died many years ago.
He is the man she loves and that's what's important and all that matters, he's great with her daughter, please try to be happy for them.
As for them moving to the US and being ' without' you, your daughter is 38 not 8 and like most people, she will probably love her new life and I'm quite sure your GD will, my brother in law lives in Florida and he and his family are very happy there.
It is unlikely that her boyfriend will choose to extend here, especially if you continue to make him unwelcome, so a choice will have to be made and your daughter will most likely, go to the US. If you do your best to hide your resentment, make him truly welcome, he will do the same for you if/ when your daughter and GD move to the US. Keep up the bitter vibes and he's not likely to go out of his way to welcome you at all, please keep this in mind especially should they have children of their own....This gentleman will be well aware of your dislike for him and unless he's a saint of a man, may not welcome you to stay in his home should they marry. Remember you're putting out bad energies towards him, this is also very unfair to your daughter and GD, he could well become her legal parent.
It's brave of you to put these thoughts on paper but I also agree with some of the others, maybe seeing a therapist if possible could be helpful, thinking and obsessing of ways to end their relationship is beyond wrong and justifying it to yourself as ' protecting your granddaughter! You are trying to protect yourself and also, it's not your place, you are not the parent here. As for the guns, we don't have to go too far in the UK to find those plus knives and machetes!
Please realise that your daughter is a grown, middle aged woman who is perfectly capable of running her own life, caring for her daughter and making her own choices. Carry on in this manner and you drive her further into his arms.
This is not about you. Please get this hatred and resentment under control, he could even put in to go back to the US earlier and take your daughter and GD with him if you continue to cause problems and you could lose all of them.
Be grateful for how lucky we are to have modern technology nowadays to keep in contact! All we had years ago were letters and a 15 minute morale call once a week and even that was timed and monitored! Now we have Skype, Facetime and Zoom!
I hope I don't sound too stern but it is kindly meant having personal experience, I hope you can come to terms with this situation, you have a lot to gain if you can and so much to lose if you can't...