Well, I'm intrigued. I can't wait to see the drawings you're having commissioned and the story if you ever feel up to sharing.
If you're going with the internal Guyver aspect, I'd put the receivers on the neck. Putting it on the back would result in issues calling the Guyver while wearing a shirt. Of course, it could almost be interesting to go one step farther. Instead of being two on the neck, it might interesting to have more than one receiver that completely rips off the clothes, resulting in something similar to Guyver IIF from the manga. It would make more sense, to me, to have multiple receivers as the whole armor coming out of two small holes on the neck is a little silly... in a way sillier than the extra dimensional stuff from the manga, I think.
I'd be interested in the changes to Gigantic. If it is an "adult" form of the Guyver, would that mean there would be no "standard" Guyver once it matured? An overdrive would be interesting and could have added issues of the effects an "overdrive" could have in terms of stamina/energy consumption resulting in a type of "overheating" scenario with a race against the clock to finish the enemy before he ran out.
Is Agito still working from within Chronos if he's a teacher? I always liked that aspect, personally.