that Owain guy looks good, no idea on the quality of his acting or screen presence, but so far he seems to look the part the best.
Molale is just.... yeah i remember his name from fancasting of yesteryear. still no
but it took The Rock this long to confirm his role, i dont think they wouldve released a statement with him as Black Adam if WB didnt already have a short list of who Captain Marvel is gonna be. given how quiet things are on that front im guessing they've already started meeting with potential actors for the part and are all under NDA.
They are not going to allow anybody with a multiple film contract jump to one of their rivals. Djimon Hounsou on the other hand is now free to play Martian Manhunter since his character is one and done in Guardians of the Galaxy.
oh now there's an idea. Hounsou as John Stewart... i can see it yup.
but to comment on your first statement-- i dont see any reason why actors from another franchise cant be hired by another franchise. this isnt the studio system anymore where they own actors. and the only contract detail i know of that limits an actor's choices in roles is the one from Marvel itself-- Marvel doesnt want reusing their actors in different roles (Chris Evans got really lucky that F4 rights belongs elsewhere) unless those roles are somewhat related (hence Paul Bettany with Jarvis and Vision)
going back to Idris Elba, i think he can make a good transition from the Thor franchise to GL. his look and sound as Heimdal is so different from his own appearance its easier to transition him into a different superhero compared to Jaime Alexander going from Lady Sif to WW.
This is the first time a villain has been cast before the hero? Funny. No matter what, The Rock will be the bigger star of the film and probably have top billing. HAHA!
didnt they cast Kevin Spacey as Luthor first, then went on to hunt for their elusive Superman (after ditching Cavill

)? its kinda a thing in Hollywood that if they have a well known villain the hero is usually an unknown