I can't see Moore playing the scene any more than I could see Adam West or Mike Myers doing it. I'm not aware of Moore ever playing a role of serious dramatic complexity and my suspicion is that if he'd made OHMSS it would have been considered the worst Bond movie either because they joked it up or because he couldn't hit any of the dramatic moments required.
Now, I await the slings and arrows of the Roger and Brosnan fans as well as those committed folks who will maintain Connery would have done a terrible job.
Have you seen Shout At The Devil (d. Peter Hunt)? Roger Moore's performance, in both this, and Gold (d. Peter Hunt), perhaps indicates how his Bond could have been had Peter Hunt directed him. Two scenes from Shout At The Devil in particular that I would draw your attention to: the burial of Sebastian's baby boy (who was ripped from his mother's arms and butchered), and the final scene where he shoots Fleischer in cold blood.
I have always thought that Roger's performance as Bond was driven more by the style of scripts and directorial choices (and mass audience demand - don't forget how enormously successful his Bond movies were) than by any (perceived) lack of acting talent.
Could Roger Moore have played Bond in OHMSS and in particular the scene where Tracy is murdered?
Absolutely!
I have seen thse films and Rog is very good in the scenes you quote. IMO, he is even better in The Man Who Haunted Himself" handling distraught, devastated grief.
BUT this was Rog in roles he respected. Many times in interviews, Rog has indicated his belief that Bond cannot be played seriously and has to be, almost, played for laughs. Rog does not believe in the realsim as Bond, whereas clearly he does as Sebastian Oldsmith, Harold Pelham and many other characters he has played.
But I do not think he would have accepted superman, lighthearted Bond being emotionally crippled at the end of OHMSS. Rog-Bond was cartoon over-dressed English sophisticate, with a permanent one-liner, a gadget to get him out of any trouble and a physical appearance which attracted women of all ages, colours and creeds.
Sadly, Rog had not respect for the raw, real emotion of Fleming Bond. He could not have done OHMSS as written.