They are called the Elementals in one of the clips so the only way we are getting Molten and Hydro man is if the Elementals find human hosts after being defeated. Mysterio does seem a lot heroish from the last clip.
In one of the links you provided, in another SFFH post, it has the director of the movie mentioning Hydro and Molten by name. So, there is that.
Spider-Man: Far From Home director Jon Watts explained during a recent set visit. “With the Vulture it’s a one-on-one fight. A mechanized villain. But with Spider-Man, you have the ability to do so much more. We’ve seen him fight much larger villains and have spectacular set pieces…I’ve always loved Hydro-Man and Molten Man and his ability to have a giant, elemental kind of creature for Spider-Man to fight against. Anything that makes things more difficult for him and opens up visual opportunities for me, I’ll run at bat.”
In ASM13, 1st appearance of Mysterio, Mysterio uses his illusions to create situations to make him appear like the hero. Mysterio is, and always will be, a con artist. This is his M.O…
See I am thinking more and more the elementals are all an illusion to cover up for Sandman, Hydroman, Molten Man.