I guess I'm a bit disappointed that this part never does get a follow-up. Man, I would be happy with a little pondering. Especially when it's revealed at the end of episode 22 that Mirai "used" to live in space, and "might" have known Hiroto. Some probing questions would not come amiss.
Did you record those voices? Why? Was that your voice in the recorder? Were you there? If so, how did you escape? What exactly happened then? Are *you* Hiroto?
But no, she might have a moment's doubt that something wasn't right, but I don't really think she would confront Mirai since...
1) Marina doesn't seem like a reflective type. She seems to live in the moment, actively moving and not dwelling in the past, so it actually makes perfect sense that she dismisses her suspicions, that is, if she has ever entertained them in the first place.
2) Those things would be too strong an accusation, even for Marina. She knows (and acknowledges) that Mirai is a "little oddball" who has a good heart and probably would never try to deliberately hurt anyone (though she seems to conveniently forget that in episode 18, which I should just leave out of this discussion).
Trying to get back to my original question somewhat, I really wonder why Mirai and Hiroto seem to have the same voice, as the scene in episode 21 strongly suggests. True, Mirai--I mean Mebius--copies Hiroto's form, but the voice Mebius has before meeting Hiroto is also sounding very much the same, assuming his voice at the very beginning of episode 1 is in real-time and not simply his reflective voice-over in Hiroto's voice after some time has passed.
Is it even possible that Mirai *is* actually Hiroto Ban? Only that amnesia, the convenient plot device I've met everywhere, made him forget, so he believes he's an alien. I've toyed with this idea more than once.
The way I look at it, perhaps Hiroto's physical body is actually dead, or missing. Mebius lends him his own body--and is content to stay in the background (i.e. very deep in Hiroto's memory) while Hiroto goes about the day, and, for some reason, not remembering that he's supposed to be Hiroto. He is determined to rescue Hiroto, whom he believes he had accidentally left behind.
Or maybe it's the other way around. Hiroto's physical body is still there...only it needs healing. Mebius heals him. However, perhaps Hiroto is too weak to recover or his soul is still trapped somewhere deep down in his own body, so Mebius, not wanting to leave Hiroto dead, leaves his own body and comes to live in Hiroto's body, so to speak. But he has all his special abilities with him. What happened in episode 21 might be simply Hiroto waking up from deep sleep and determined to get Mebius' body back. Either way, Hiroto is still with Mirai, one way or the other.
...Or perhaps it's something so complicated, and I'm seeing conspiracy where none exists. Hiroto is actually a human who is made into Ultraman, for some reason or other (but most probably because of his bravary in staying behind). This involves too much mind washing and bending truth and I can't really see why (actually I don't believe) Ultras would deliberately do something this cruel (erasing his existence, so to speak) to another sentient being, especially to someone as good as Hiroto. But, on the other hand, I don't recall seeing anything of Mebius (in the series, don't ask me about all the side/post story stuff I haven't laid eyes on) until just right before his arrival on Earth. What I mean is, sad as it is, it doesn't look to me like Mebius really exists before the start of the series. And how much of Mirai's memory can be trusted anyway? His reality (as he believes) can be made up, for all I know.
With all that said, I actually don't know what to think. Perhaps the Mebi writers forgot about the inconsistency. Then again, they didn't seem to have any trouble keeping Hikari and Serizawa apart, at least by their character's very distinct voices. (Hey, I still don't think these two are very distinguishable from one another otherwise, especially in later episodes, but that's old news.)
EDIT: If you miss the first, more sensible Mebius 21-22 review (and the many interesting, thoughtful comments), it's here. I think most of the content of this post is too much of my fantasy anyway.