the problem you experience is exactly why navigation does not work 
with the default behaviour in the first place.

The problem is that in iPhoto '05, if you tell it to start a 
slideshow via AppleScript, this command does not finish until the 
actual slideshow does finish. Therefore, an "async" option was 
introduced in iPhoto '06, which Remote Buddy is also using.

Now, if you start the slideshow via AppleScript, even if you tell 
AppleScript not to wait for a reply, iPhoto will still block and not 
accept any AppleEvents sent by AppleScript until the slideshow has 
exited.

That's why, if you execute that AppleScript, the events it sends will 
not get executed until after the slideshow has exited. At that point, 
however, it does nothing, as the slideshow is no longer running anyway.

So, while you can only use AppleScript to start a presentation 
(unless you start it manually by mouse through the UI), you can use a 
keystroke to exit from it ("Esc" should work - and you can just lend 
an action that does just that from the Virtual Keyboard).

Hope that helps.

Best regards, 
Felix