Rip,
Legally, I believe you have a right to have access to the marital home at this stage. From a DB standpoint, I'm not sure what the right move is. I can see the figurative meaning in giving up the key.

As far as keys to your new place, I don't think you need to give STBX a key. That's your place. If she needs to get some of the kids things, she can make arrangements with you, just like a friend or neighbor would.

Me:35 W:30
D:4 S:1
Bomb: 01/08/15, discovered EA & PA
In House Separation: 01/14/15
W moves out: 04/05/15
I tell OM's W about A: 04/15/15
W serves D papers: 06/19/15
Mediation: 09/16/15
D final: 12/01/15