Upfront: I have never used the SDK

I am a co-developer of a flight simulation application running on Linux, Max, and Win (x86 and x64). I want to use TS a voice server for ATC communication. Basically I want to open a channel for each frequency contacted and assign the user to that channel. If the user contacts another frequency it joins the other channel, or creates the channel first if not existing.

Just to head in the right direction I do want to clarify some questions.

a) Do I need to use the SDK for that, or can I just use some kind of URL schema.
b) If I need the SDK, can I create code which runs on the above 3 Platforms Win, Mac, Linux
c) When I create such a TS integration, does it require to have TS installed or will it run "self providing"
d) We have our own Push-To Talk key, can we use that or is the TS PTT key independent from that.

As said, I have never used the SDK so I have some very basic questions just to head in the right direction.