HaWho, I am sure the lawyer you choose will give the correct info about the separation date. I don’t think it could be any date for your H to declare. It has to have some legal weight. I’m almost 100% positive that it will be the date your H took an action, either consulting with the lawyer or filing for the D, not the date he had in his sick head. I believe there are some provisions in our state about the benefits you get after certain number of years, and I believe it is at least 10 years.
Your situation is different from others. I agree with advice to get what you can right of the bet, while your H feels an urgency to get out and also feels some guilt. I think I did a smart move when my H said that he didn’t want the house and could have it, so I refied to my name and he singed a paper giving the house to me. I think later (in a year or two) he regretted it, but… it was too late  , I have the house now. It could be different with your H, since he’s been brewing slowly in his dorm room… If you can get the best deal for you, go for it. If not, you could wait. It will cost you more money for the lawyer, but if it would have a better financial outcome for you, it would be worth it.
M:50 H:52 S28 (my S from previous marriage) M:17 + 3 BD: 06/12 S: 06/12 - H works in another state