Not to sure about fiat to BTC and back, but there are definitely some interesting projects working on decentralised exchange of altcoins.  Qora, Burst and a few other altcoins are working on AT, but it's difficult to predict if or when this method will find a home within BTC itself.  You'll be able to jump from one blockchain to another without buying or selling on centralised exchanges.

Haven't specifically talked about it, but I'm pretty sure there are few Decentralized exchanges out there.

has someone tested bitsquare?
it looks they are already running.

coinfeinne is a solid future project, I am looking the project really close because it can change the bitcoin world as we know it now

It is really not possible to have a truly decentralized altcoin exchange. There are simply not enough volume for altcoins for this to be possible on any large scale. Plus any decentralized exchange would need to involve one person sending funds to the other first which would result in the potential for scamming