To be honest OP, you seem as much hung up on his DW as he is.
You offered to cancel the date on the anniversary twice, no doubt making it seem as though you expected him to be distracted by her. Maybe he would have, but maybe he was planning a bit of a wallow during the day followed by a nice evening with his GF to distract him and to look to the future? You made assumptions about how he would feel on the anniversary, rather than letting him do it in his own way without any expectation from you,
Now you keep questioning who he's with. He's meeting up with old friends, yes, I agree it would be nice to have invited you to the group event. I'd be hurt if I were you. But the wife's best friend catch up is surely him meeting up with and old friend? Isn't he entitled to see people from his past? Surely it'd be stranger and less healthy if he eliminated all links to his wife?
But the screensave would be a no from me.
FWIW, my DM died when I was a baby. My DF apparently adored her, idolised her. But was remarried within 18 months and has lived happily with his second wife for 45 years. No mention of my DM is ever made, no anniversaries are marked, no photos, anecdotes, momentoes.
People mourn differently. Not everyone follows a pattern of grief. If you think you want to continue with him, let him deal with it in his way and respect him if he says he wants to be with you.