Phoenix fist is used in a lot of ways, if you crack the sternum it wont kill them (probably, could always have a hart condition I guess), pain is not the worst you can do to them. Some Chin Na could really damage them long term if they resist.
Having been on the receiving end of an attack I say ethics come before and after the fight, in victory there is life, if you lose you are at their mercy and I trust my ethics more, I wont stamp on their head, in the UK that is how most fights seem to end. Hint, its not fun being on the receiving end, holding back will cost you and its easy to trip up even without their help, then they will pay you for your consideration. End the fight, go home be happy.
I am a advocate of Karma, if they attack you and get injured its as if you were a wall they hit them self against. Don't blame the wall, never fight unless you have to!
I advise sticking with Mantis as you love it but studying Chin Na as well as more options are good.
As for other styles, why not Taiji, it has a large range of options, look at the Taiji fighting set clip on this site.
Don't get me wrong I admire your compassion but remember it was Buddhists who invented most kung fu. Having the option to really mess someone up hardly means you will.