KLANG would not fix any of your jack problems, anymore than using OSS, ALSA, Coreaudio, FFADO and portaudio backends would address them. they are backends - they don't directly interact with Jack's routing and clients/apps. As far as not knowing how to get started, maybe jackaudio.org and linuxaudio.org might help. There are a ton of tools, apps and options for jack and also documentation. Jack it not that hard to setup and there are many good apps/tools for managing it. My first experiences (back 5 years ago) were pretty good, i did have to read a little doumentation - but that should be expected, when we are talking about proaudio systems (analog or digital). After getting started, i haven't looked back.
as far as using DSP processing of some kind of global outputs, i do that right now (and have been for years) using Jack. I use Jackdbus as my soundserver (no PA) and manage my sessions via Gladish. In my case, i own a nice commercial mastering suite (for windows/mac) called 'T-Racks', which i run via Wine + FSThost. My setup is overkill for a general desktop user, but i am a musician and thus want to be able to sit in front of one of my midi keyboards and be able to play music, at any moment in time... maybe take a quick piano lesson online, as well as having access to multi-gig sample libraries (think pianos) as well as top-notch VSTi/plugins (Organ3, Absynth, etc). but even without such high demands (my system is low-latency, RT kernel 64-128frames / 96khz multi-in/out), i could run Jack with a slightly couple of LV2 plugins on the master and play with FOSS synths/apps. ~ which the reality is, if using linux for proaudio - you are going to be using jack, not ALSA or PA.
but even with PA - couldn't someone just write a compressor/limiter/leveling amplifier for PA? (in some mixer utility) this hardly sounds like a case, where we need a complete re-write of both kernel and user-space sound APIs to accomplish... if i remember correctly, isn't it also possible to do this with ALSA/dmix too?