1. In terms of in-game stuff, I am just guessing that you have your in-game macros configured with your targeting/assisting built into them. With ISBoxer, you usually want your macros to be the same as they would be when playing the character solo, and whatever method of targeting can be performed outside of the macro. (Because the assist button can get pressed for you, before your macro) This is what you would really be changing. You can use your existing macros if you want, either way. You don't have to switch to ISBoxer's targeting/assisting methods, but using ISBoxer's method lets you interchange different characters in different teams easier (i.e. without changing all of your macros in the game).

2. One license (subscription) allows use on up to 5 PCs. Some multiboxers have 6, that's fine -- the limitation is to help prevent abuse not to force you to buy another license just to add that 6th PC.

3. ISBoxer's macros are designed to not affect anything when you're not multiboxing. That's because the macros are installed by the ISBoxer Addon (created by ISBoxer on demand), and the ISBoxer Addon isn't going to interfere with anything if you're not launching through ISBoxer.

4.There's plenty of threads with answers from lots of people on this one if you're worried about me injecting bias in the answer (googling keyclone vs isboxer comes up with http://www.dual-boxing.com/showthread.php?t=31275 from 2010 and http://www.dual-boxing.com/showthread.php?t=23192 from 2009)... but the most common example is real-time mouse broadcasting -- Try dropping a Hurricane, Blizzard, Death and Decay, Rain of Fire or Earthquake with Keyclone. The other most common answer is support -- Rob hasn't posted on dual-boxing.com since last December (his profile: http://www.dual-boxing.com/member.php?u=774). Conversely, I develop and support ISBoxer full time and I've been selling multiboxing software since 2004.

Although honestly if you're looking for people to give you reasons why you should use Keyclone rather than ISBoxer, you're probably going to think everyone's answers are biased...