Just finished a game as Unfallen, which was pretty cool using Peace as my main aggressive status.

I used both Pacific Conversion and Force Peace to great effect, but had some questions:

* I noticed that once I research the appropriate tech (SLC systems I think), "enemy" systems under my influence would get a bar with the peace symbol.  This bar slowly filled up to 100%, which made it cheaper and cheaper to convert them with influence.  Most systems seemed to gain around 2%~6% per turn, but I couldn't figure out what caused that?  Does anybody know what factors increase / decrease the rate of conversion?  Possibly how much influence that system is generating, or how many improvements are in it?

* Using Force Peace typically seemed to be a neat, though expensive, option for causing Peace.  However, eventually I got 2 Heroes who each had a "-50% to cost of Peace/War treaties while on Senate" skill.  Apparently it gave me a combined -100% to cost of Peace/War treaties, because thereafter it was always free for me to declare War, Force Peace, etc.  Unless I'm missing something, this seems comepletely overpowered / broken / unbalanced?  I was playing as Unfallen, which thrives in Peace, and yet no other empire could hope to ever attack me, since anytime they declared War (set to Cold War) I could just Force Peace them for free.  Especially once I got the Pacific Conversion tech, I could keep converting their systems, and if they got upset and declared War (set to Cold War), just Force Peace for free.  Am I missing something, or is Pacifist Leading Party for Force Peace with -100% Peace treaty cost for a Peaceful playstyle a completely OP combo?

Sounds like you may have found a exploit. I would be curious to play a save from the second half of this game if you have one. Playing a unfallen game on endless speed endless ai 12 ways now for practice.  

This does sound like it is not intended, you should never be in a position to for peace for free. The only diplomatic action that involves a war or peace should be the cravers war declaration after there quest line completion.

Whoops, looks like a very large factor I failed to consider is that the AI was usually just using Cold War on me, not a full and proper Declare War.  It seems that Force Peace is only available in Cold War, not full War, so this feature is not nearly as powerful as I first thought, at least against other humans who can just Declare War to avoid it.

Against AI however, this may be still be quite powerful since it at least appears that the AI is reluctant to ever go fully from peace to Declare War in a single turn.  Currently if they always go through an intermediate Cold War Phase, I can always Force Peace them back to Peace before they ever get to full War.  E.G. in my game I would use Pacific Conversion on a colony, the AI would get upset and go to Cold War, I would Force Peace, then capture the next system, etc. over and over and they wouldn't fully Declare War.

Here are some of the later saves in that game, before I felt I understood enough and just captured all the home systems.

Unf 4.sav

Unf 5.sav

Unf 6.sav

Unf 7.sav

Unf 8.sav

Unf 8a.sav