For the theory, I like it... but... I don't believe the one at the end qualifies as an actual ghost because.
1. I'm pretty sure if the story continued, it would turn out to be just someone else messing with them and not an actual ghost. That was basically the theme of the episode... some characters think there's a ghost... it turns out to be someone else... and that someone else ends up being pranked by someone else etc.
2. NRDD is not a supernatural/fantasy show. So, things like ghosts, zombies, superpowers can't be canon.
3. In NRDD universe, The Thundermans is a show which means the Haunted Hathaways is a show too because Thundermans and Haunted Hathaways are in the same universe. So, their characters can't show up on NRDD unless it's like a dream like Dawn's dream in The Wonderful Wizard of Quads where they had Dr. Colosso.
Some shows might not care about mixing supernatural elements or characters from other universes into their show... but NRDD writers seemed to be very careful about the rules.
If we forget about the rules for a second, I would actually say Miles is the same Miles from Haunted Hathaways. He had the ability to change his appearance and look like someone else. So, maybe, he still kept his Miles name but changed his appearance.