Nice job on this review, Ian. I like how you explained the moment when you knew it would get four stars from you. Some of the things you mentioned, I wanted to comment on more specifically:
The mirror/flashback scene. I alluded to this in my review, and I think symbolically it represents the way anxieties about war are reflected back on the characters (and viewers, too). This is the kind of visual trick that someone like Douglas Sirk would perfect in his melodramas of the 1950s. Sometimes Sirk would accomplish it with windows or other objects with reflecting surfaces.
The Invisible Man comment. This is where you went in a different direction than me, and I find that fascinating. TOMORROW IS FOREVER does have elements of horror, which Welles pulls off superbly, without really veering into cheesiness or camp.
Keeping secrets. Not only does Kessler (Welles' Austrian character) keep the secret of his being alive from his wife, but just as importantly, the secret of who he is and his overall identity is kept from their son. After Kessler's death, we can guess that Colbert will still consider him to have been John her first husband and you can't help but wonder if she will always keep those thoughts from the son. If so, then she becomes the keeper of the secret. The scene in the train station where Kessler acts very paternalistic without admitting he's Long's real father is very powerful.
The scene where the music is absent allows Welles and Colbert to play as if it were a scene in a play. I am glad you highlighted that moment for people watching your review. To me, it proves how fine an actress Claudette Colbert was, holding her own with Welles.
Some trivia: Welles and Colbert both worked with Long again, in other films from this period. Long played Welles' young brother-in-law in 1946's THE STRANGER. And the following year in Universal's rom-com THE EGG AND I, Long had some nice scenes with Colbert. That time he did not play her son, but the son of a raucous neighbor woman (Marjorie Main).
Colbert never had children of her own. Years later, when Natalie Wood died, Colbert was in advanced age and very distraught, claiming it felt like her own daughter had died. In an audio commentary for one of his movies, Robert Wagner (husband of Natalie Wood) talks about how Claudette was a mother to all of the younger stars on the set of LET'S MAKE IT LEGAL. He also said that if anyone should have had children in real life, it should have been Claudette Colbert.
So I think the actress bonded well with the kids in her later movies, and it shows. It was genuine, which makes her performances seem more realistic and natural. This provides a nice complement to someone like George Brent who does not exactly emote in his films, as well as an obvious contrast to Welles in TOMORROW IS FOREVER, whose somewhat grandiose technique relies on an accent, make-up and other wardrobe gimmicks.