Mieu:
Look! A spaceship that's just like ours! It must be
the other surviving spaceship.
Wren:
That spaceship is sending us a message.
Voice:
Hello Alisa III. This is the spaceship Neo Palm. That
explosion earlier attracted our attention. We guessed
that Dark Force had arisen again and we came over to
lend any help you needed. Since you're still here, we
assume you've won. In 1,000 years, Dark Force will arise
again. Let's get rid of it for good next time!
From this text...it doesn't say that they actually defeated Dark Force, but perhaps they somehow knew about it? Besides, I'm sure that the Japanese version's different.
As for the initial question, I believe that Dark Force is individual; merely a creation of the PD. It has its own consciousness, right? Like when it's under the mascarade as Seth, it manipulated Chaz and the others into letting him join, only to use them to get ahold of the Aero Prism. And in PSII when you see it, it seems to mock Rolf and the others (as it also does in PSIII with the 3rd generation heroes).
Its actions throughout the series are conducted with the intent of freeing the PD; is this a puppet under the PD's control? I'd like to think this is and isn't true, and therein lies a conflicting answer. As a puppet, it's being used for the goal of freeing its' creator, but I think that is to the extent of which it goes. As for its own desires, it yearns for the destruction of the GL's creations.
I think there
can be more than one Dark Force, and who says that Dark Force can't survive without the PD? Would that mean that, if the GL was actually gone, then would Algo fall to ruin and just vanish? Then again, that
would be the case if it were a manifestation...
Take this example. Leonardo Da Vinchi painted many paintings, and one that I'm sure we all know about is the famous Mona Lisa. Even after his death, it still exists. To say that it wouldn't exist after his death simply
makes no sense since it's a physical object. It can still be destroyed and all, but it hasn't yet it's still around.
With that in mind, Dark Force could be like a painting, created by the PD... When the GL's flames (hah, fire...but really, it's an example of protectors!) burn the dark painting, it turns to ashes, hence being destroyed. So why couldn't the PD paint more pictures, or so to speak? Or more than one, for that matter?
This is where I think the seal comes in... I don't really believe that the PD was really destroyed in PSIV, but the seal was just fixed with the Elsydeon. Could the protectors really destroy something that the GL had a hard time with? I somehow doubt it.
I don't think I have anything more to add to this at the moment...