I applaud your "Look before your leap" attitude. With 1-click solutions and such, people tend to root and rummage around in their devices, without understanding whats going on and just how easily things can get mucked up.
I'm no expert on the matter, so anyone who see any incorrect info, feel free to call me out on it. that beings said..
1. I think the Eng-s off deals with your boot loader. This allows you phone to run unsigned firmwares. The official firmwares from manufacturers come signed.
1b. I think the Radio-s off allows you to install other roms.
2. Clockwork recovery is an enhanced version of the default phone recovery system. Clockwork allows you to make a total backup copy of your device. The backup will return you device to the exact status that it was when you made the copy. You will also gain access to other phone operations that you normally wouldn't have.
3. I'm not familiar with this Rom, but I think that what you might be reading only pertains to installing an updated version of this Rom on top of itself. When you install a new Rom for the first time, you will usually have to do a total wipe of your phone. Any apps or information that your have will be erased.
4. I have all my contacts synced to Google. In case of a factory reset, or if I change my Rom, my contacts are automatically restored when I reconnect to my Google account. You can also make a physical copy on you SD card. Inside of your contacts menu, there should be an option to export your contacts. They will be save to your memory card.
Here is a link to the sub forums fro your phone.
HTC Inspire 4G - Android Forums
Hope this helps