My take on plot SPOILER
I've been reading a lot of your reviews. Unless my husband and I got it all wrong, Love, Death, and Time were not real people. Like in the hospital scene when Death was sitting next to the ex-wife and told her about Collateral Beauty, comforting her about losing her daughter.
Love appeared to Whit and led him to the 'theater' where he 'met' the others. They only appeared to the 3 friends and to Howard. His wife couldn't see them when she looked back on the bridge even though she had seen Death at the hospital, when she needed support. They were there to say good-bye to Howard. The photographer couldn't see them so there was nothing to remove from the pictures.
Love, Death, and Time not only helped Howard recover from his grief, but helped the friends as well. Whit (Love - to mend his relationship with his daughter), Claire (Time - whether or not to have a baby) and Simon (Death - to tell his wife so they grieve together). I'd like to see it again to pick up on all the hints.
Let me know what you think.