Hi Svarl1,
I'm sorry that it has come to this. Deciding to leave a partner is really tough, and you sound like you have reflected and planned well. I commend you for taking this step by step and planning your departure in a way that will cause the least stress for you both.
I don't have experience in this kind of exit plan, but if there is any chance of violence or suicide threat, I think your idea of a third person is a wise one.
Here are some resources that you may not have seen that will be helpful in your plan, and for the handling of communication afterward:
Leaving a Partner with Borderline Personality DisorderBIFF TechniqueKarpman Drama TriangleI look forward to reading others' perspectives and experiences.
heartandwhole