What is the difference between an "iOS App Development" certificate and an "In-House and Ad Hoc" one?
Source of everything I'm about to say: iOS developership.
First of all...
You can not deploy a development app on a second device without using another one of your 100 devices. You just can't.
Now, the list of questions.

Yes, you can run development apps that haven't been signed with a distribution certificate.
No. The development cert may only be installed on registered devices.
Yes, if you are serious about development. You could get by with only an ad-hoc cert, but it would be cumbersome, and you wouldn't be able to use hugely helpful Xcode debugging features like Breakpoints and logging.
I don't know, but I would assume so.