Nope, you describe it accurately.
There wouldn't be another setup fee, no, since your account's already set up and activated and what have you. You would, however, be paying the difference in price between the two accounts. Although, unless you're planning on taking advantage of the small differences offered by Code Monster, CDI will probably suit you just fine as it literally grows with you.
If by account you mean shell user, you can have up to the maximum allowed by your current plan (CDI is, I believe, 75). You can have multiple control panel accounts set up as priveleged users for one or more of your plans if you want, but if it's just you using it that would be a bit impractical. Basicly, the only thing you can't have is more than one billing account.
Nope. DH would catch you, probably kill one or both your accounts, and you wouldn't get a thing out of it. Besides that, see the answer to your last question.
Yes, I have an opinion.
Get a minimum 50% off with the "haveadreamyday" promo code, and... have a dreamy day. Original, no?