Chat as a Service (CAAS) Solutions

We are currently in closed-beta with our Qt/C++ based video application. Our users are requesting "text-chat" as a way to communicate with other users.

Knowing the litany of "Chat" products that are out there, I'm not a huge fan of redundancy in communication products. I figure this must be a SaaS that completes my goals. Furthermore, many of these products are "mobile first" (as in they require SMS verification to begin using their service).

Things I'm looking for:

- I'd prefer not to maintain servers

- Must have a RESTful API or C++ SDK (we are not developing for Android / iOS independently) and we are not building a website (ie. JS SDKs are not very useful)

- Ability to add and remove users programmatically when they create accounts on our service.

- Security (obviously)

- Would prefer not to warehouse messages or be able to access them

Things I've looked at:

Open Whisper Systems "Signal" Protocol.

Pros:

- Known standard for secure communications.

Cons:

- Signal "Text Secure Server" code is unwieldy and poorly documented (https://github.com/WhisperSystems/Signal-Server)

Twilio "Programmable Chat"

Pros:

- Dynamic REST APIs

- 3rd Party hosted (don't have any servers to maintain and scale)

Cons:

- Corporate Service hosting (I know Twilio is better than most)

Thoughts?