extremerotary

@AlexKazoo
Hey Alex,
Update the system schema file. I put a PR in for this a few months ago - i'll have to check on its status...
system_schemas/media

I don't know how this translates to the advanced provisioner, so hopefully another member can give some instruction, but below are the configuration parameters you'll want to use. These enable the device to use the replaces header in a directed-call pickup situation when the BLF is ringing. So, from a use case:
You are watching extension 101 on your device. A call comes in and rings extension 101. Your BLF lamp is flashing for 101. You push the BLF key and intercept the call that was ringing 101.
<call call.directedCallPickupString="" call.directedCallPickupMethod="native" /> <attendant attendant.resourceList.1.address="{{ line_key.extension_number }}" attendant.resourceList.1.callAddress="{{ line_key.extension_number }}" attendant.resourceList.1.label="{{ line_key.label }}" attendant.resourceList.1.type="automata" />  

@martin
Welcome back!
There have been significant improvements in Kazoo in the past few years. Check out the doc site for starters. There are some tutorials on system installation of the 4.1 version, and a lot, lot more documentation on the APIs and other inter-workings of the application. It's a developer's best tool!
https://docs.2600hz.com/sysadmin/doc/intro/about/
I absolutely love your enthusiasm and willingness to contribute; that goes really far around here. Unfortunately I'm not able to answer a lot of those questions because I'm not a hosted customer of 2600hz, and I don't know the issues that you were running to in the past. 
From my experience working with their APIs, everything that you've listed in bullets 1 and 2 is working. Call rating I'm a little fuzzy on because we don't use that particular feature. The limits in jonny5 work great. 
I have no experience with braintree. 
I can't speak to 2600's position on carriers, so those questions I'll leave to @Darren Schreiber
As for documentation - I guess it depends what kind you are interested in creating. That will determine the best place for them, and I know Darren can also assist with that question. Great to have you back in the community! I look forward to helping you out where I can!