1. No, you do not need to. However, it sounds like you are more familiar with your previous method and less confused by it, so you can always use a Keystroke Action instead of a WoW Macro Action, and use your old way if you want.
2. Usually, yes. You would want to make a Key Map with a Mapped Key in it to disable other Key Maps. Zanthor's guide shows how to do it, or the Quick Setup Wizard in ISBoxer 33 will generate such a key for you that you can use as an example.
3. Ideally, you want the same keys to perform the same actions, no matter which character you're playing. The Mapped Keys will work FROM whichever character is assigned to it, and will send keys TO whichever are configured as Targets in the Actions. So you could make a key that only works FROM one or the other by making a key map for each character separately. You would make a key that sends different instructions TO each character by having multiple Actions, or by using ISBoxer's condition processing in a WoW Macro Action as you mention -- but you do NOT need do use that. The purpose of the variables and condition processing is to automatically generate a different macro depending on the characters -- you can make them manually if it's over your head.

Your macro could be shortened to this:
Code:
/targetexact charactername
/cast Renew
The WoW Macro Action variables dont let you easily make multiple hotkeys the way you're asking -- you would simply need to have a macro like this:
Code:
/cast [target=party1] Renew
for each party member. this can be done with a WoW Macro Action, or use a Keystroke Action that does a wow macro that was created a different way (bind pad, etc)